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ASTRONOMY 

FOR 

SCHOOLS 



Saturn and its Rings, as discovered in 1838. See page 147. 



ASTRONOMY 

FOR 

SCHOOLS; 

UPON THE BASIS OF 

MONS. ARAGO'S LECTURES 
AT THE ROYAL OBSERVATORY OF PARIS; 

AND IN WHICH THE LEADING TRUTHS OF THAT SCIENCE 
ARE CLEARLY ILLUSTRATED, 

WITHOUT 

MATHEMATICAL DEMONSTRATIONS: 

WITH NUMEROUS ENGRAVINGS, 



APPENDIX 




BY R. W. HASKINS, A. M. 



NEW YORK: 
robinson, pratt & co., 63 wall st. 
buffalo: a. w. wilgus. 
1841. 



<?8<f4- 
<Afc3 



Entered according to act of Congress, in the year 18-11, by 
ROSWELL W. HASKINS, 

in the Clerk's Office of the Northern District of New- York. 



D -t 



Wilgus' Pre.-s. Ruflalo. 



/9? 



PREFACE. 



The basis of the following work, as announced in the title page, 
re Arago's Lectures at the Observatory of Paris. Many changes 
and digressions from that author have, of course, been necessary, 
in preparing a work lor the schools of the United States, and suited 
to the wants, and the peculiar paucity of knowledge, upon this par- 
ticular science, among the greater part of our American youth. 

It has often been matter of surprise, and still more frequently 
of poignant regret, that Astronomy, although the most ancient 
and the most perfect of all the sciences, should never have been 
generally introduced into the list of subjects embraced by the first 
degrees of publick instruction, among us. When the same fact 
has been complainingly averred in Europe, it has been answered 
that this science has been, through all former time, exclusively con- 
fined to the learned, because the books, upon it, have been so loaded 
with difficulties as to render access to the science impossible to 
any others than such as were deeply versed in Mathematicks. 

In the lectures of Arago, referred to, above, it was the assumed 
task of that eminent Astronomer to prepare a work suited to gene- 
ral use, by giving it an elementary form, without sacrificing any 
thing essential to a general knowledge of the science; and whate- 
ver of success may have crowned his efforts, in this, the present 
work will be found to embrace. 

The almost entire absence of the study of this science, from our 
common schools, and the constant and daily increasing demand for 
an Astronomy suitable to be used as a Text Book in these schools, 
are matters of perfect familiarity to all such as have given any con- 
siderable attention to the subject of publick instruction, and its pro- 
gress, in the schools in question. 
1* 



VI PREFACE. 

The rising character of these schools is a proud and distinguish- 
ing feature of the present era. The middle aged, of the present 
day, well remember the first general introduction of the study of 
geography into our publick schools: now, perhaps, no corner of our 
land, however obscure, can be found where the schools do not em- 
brace this study, even in laborious detail. 

The study of geography has prepared the way for that of Astro- 
nomy, of which, indeed, it is, in some sort, a subordinate branch; 
and which can be well understood only in combination with a 
knowledge of the mechanism of the heavens. 

In the preparation of this work the constant endeavour has been 
to meet the demand of our common schools, for a Text Book in 
Astronomy; and to give it general application by suiting it to the 
capacity of all those who have embraced, in their course of studies, 
geography, and common arithmetick. For this reason no mathe- 
matical problems, whatever, have been inserted; and while it is 
confidently believed that the work may be fully comprehended by 
pupils no farther advanced than to embrace the studies above men- 
tioned, it is as confidently hoped that it will be found to include all 
that is essential to the general pupil in the science. 

We have said this science is ancient. It is so: originating in 
the fields, and among peasants, at a period so early as to be now 
unknown, it has successively passed from men thus untaught, to 
minds the most exalted and sublime. Imposing, by the grandeur 
of its objects; curious, by its peculiar means of investigation; and 
astonishing by the number and character of its discoveries, it 
marks, perhaps, more pointedly than any other science, the extent 
to which human intelligence is capable of fathoming the secrets 
and the resources of Nature. But, though ancient, in its rude out- 
line, yet perfected astronomy is of very modern date. Though 
many of the great truths of the science were disclosed by the im- 
mortal spirits of previous days, yet it is only about one hundred and 
twenty years since the true planetary adjustments and motions of 
the Solar System were taught in the colleges and learned societies 
of Europe. And yet, such was the rapid march of the science, after 
that period, that "at the close of the last century there did not re- 
main a single phenomenon, in the celestial motions, that was not 
explained, on the principle of gravitation." 



PREFACE. VH 

This minute knowledge, thus perfected only some forty years 
since, was, of course, confined to the learned; nor is it supposed 
that others than such will ever attempt to become masters of these 
abstruse demonstrations. But the spirit of this age, no less than 
the salutary advantages of the knowledge itself, demands that the 
results thus discovered, and the general laws by which these results 
are produced, should become part of the instruction imparted by 
our schools, to the youth for whose benefit they are maintained. 
Such knowledge will be a valuable substitute for the lunar, stel- 
lary and cometary superstitions, the remnants of that malady of 
the human mind, ancient astrology, which are so widely diffused, 
and which can only be ejected from the mind by a substitution of 
true knowledge in their stead. To this end no study so directly 
conduces as Astronomy. "The objects which it treats of," says 
Playfair, "cannot fail to impart to it a degree of their own magnifi- 
cence and splendour; while their distance, their magnitude, the 
steadiness and regularity of their movements, deeply impress the 
imagination, and afford a noble exercise to the understanding. 
Add to this, that the history of Astronomy is that w r hich is best 
marked out in the progress of human knowledge. Through the 
darkness of the early ages, we perceive the truths of this science 
shining, as it were, by their own light, and scattering some rays 
around them, that serve to discover a few definite objects, amid the 
confusion of ancient tradition — a few fixed points amid the uncer- 
tainty of Greek, Egyptian, or even Hindoo mythology. ,, 

Perfection, of course, I have not hoped to attain; yet great 
labour has been bestowed, by a constant reference to first authori- 
ties, to ensure, as far as practicable, an avoidance of errour3 w T hich 
are known to abound in minor compilations. 

Every effort, also, has been made, and it is believed successfully, 
to obtain, from the great Astronomical depositories of Europe, full 
and accurate knowledge of all the recent discoveries in the science: 
and the results of these efforts will be found occupying their appro- 
priate places, in the different parts of the work to wrhich they seve- 
rally pertain. 

My hope is that this work may become instrumental in impart- 
ing to our youth, in the publick schools, a general knowledge of 
the leading truths of Astronomy: of that sublime science which 



Vlll PREFACE. 

has extended man's views, upon every side; and, by rendering this 
globe too narrow a space for the range of the human mind, has 
enlarged its conceptions of the extent, no less than its knowledge 
of the mechanism of the Universe: of that science which is identi- 
fied with our daily and hourly necessities; which regulates the 
labours of agriculture; affords us the only measure of time that we 
have, or can obtain; is inseparable from accurate geographical 
knowledge, and is indispensable to navigation: and should it prove 
useful in this, then, indeed, the end for which it was undertaken 
will have been fully -attained. 

THE AUTHOR. 
Buffalo, May 10, 1841. 



INDEX 



Page- 

Acromatick Telescopes, 29 

Aerolites, 119 

Asia, depression of land in, how produced, 2*22 

Aspect of the heavens 67 

Astronomers, the Chaldeans among the first, .... 41 

the Phenecian, 41 

the Greek, 41 

Astronomy, history of, 40 

carried to Greece by Thales, 41 

taught by Anaxamander, 42 

taught by Pythagoras, 42 

sought for in Egypt, 43 

taught by Aristotle, 43 

" at the school of Alexandria, ... 43 

" by Ptolemy, 44 

" by Copernicus, 44 

" by Kepler, 45 

" by Galileo, 45 

Astronomical terms, explanation of, 319 

Alphabet, Greek, 47 

Annual parallaxes, table of, 150 

Authorities, 15 

Atmosphere, of the, 252 

height of the, 253 

refraction caused by, 253 

Bode's law of planetary distances, 100 

supposed solution of, 100 

Breeze, land, and sea, 280 

Calendar, of the, 281 

Celestial bodies, apparent motion of the, 67 

sphere, reflections upon the motion of the, . 69 

Ceres, 135 



X INDEX. 

Chronometer, how to be set, for finding longitude, . . 248 

Comet, parts of defined, 189 

of 1680, . . . ' 189 

ofl?59, 192 

of 1770, 194 

of a short period, 195 

of six years and three quarters, .... 195 

of 1744, with six tails, 199 

Halley's, strange appearance of, at the Cape of 

Good Hope, . - 201 

Halley's, as seen at Buffalo, 197 

can the earth pass into the tail of a, . . . 210 
were the dry fogs of 1783 and 1831 caused 

by the tail of a, 211 

has the moon been struck by a, . • . . 213 

has the moon been formerly a, 213 

is it possible that the earth may become a satel- 
lite of a, 214 

have the lattudes of places upon the earth been 

changed by a blow from a, .... 219 

Comets, 69 

physical constitution of, 196 

the tails, different directions of, ... 198 
are they self luminous 1 ? curious experiment to 

determine this question, 203 

have they a sensible influence upon the course of 

the seasons? 205 

can one of them strike the earth? .... 206 

has one of them ever struck the earth? . . 208 

Colours, complementary, 39 

some persons unable to distinguish, ... 40 

Continents, western coasts of coldest, 272 

Cycle, solar, 287 

Days and seasons, of the, 264 

Day, sidereal, 70 

Definitions, 46 

Densities of the sun and planets, table of the, .... 155 

Distances of the sun and planets, table of the, .... 154 

Distance passed over by each planet, per minute, . • . 157 

Discovery curious, explained, 221 

Earth, figure of the, 167 

dimensions of the, 168 

diurnal rotation of the, 171 

cause of the shape of the, 177 

annual motion of the, 178 



INDEX. XI 

Earth, two motions of the, 270 

temperature of the, "271 

mean tempeature of, at the equator, . . . 272 

Ecliptick, diminution of the inclination of, 188 

Eclipses, of 224 

of the moon, 224 

of the sun, 227 

" total one of June, 1806, ... 231 

ancient ones, their aids to chronology, . . 232 

Ellipse, 49 

how to describe one, the foci and the transverse 

axis being given, 53 

how to describe, the axes, only, being given, 53 

Eye. conformation of the, 35 

Equinoxes, precession of the, 185 

Gulf stream, how caused, 243 

Heavens, aspect of the, varies with the position of the observer, 73 

Heat, has the earth primitive? 273 

Inclination of orbits to the ecliptick, table of the, . . . 157 
axes of sun and planets to their respective or- 
bits, table of the, 157 

Inequalities, secular and periodiek, 183 

lunar and terrestrial, 183 

Juno, 135 

Jupiter, 137 

satellites of, 138 

physical constitution of, 140 

Laws of Kepler, 158 

Latitude and Longitude, determination of, 243 

Lenses, of, 23 

property of, useful for lighthouses, ... 25 

Light, general laws of the reflection of, 17 

reflection of, by plane mirrors, 19 

concave " 21 

convex " 22 

general laws of the refraction of, ... . 22 

dispersion of, 23 

calorifick and chymical rays of, ... . 23 
decomposed, in rays of different colours, when 

refracted, 28 

progressive motion of, 175 

abberration of, 180 

Longitude, finding of, difficult, 245 



Xll INDEX. 

Longitude, method of finding, by chronometer, and astro- 
nomical tables, 246 

interesting account of instance of finding, 247 
tables for finding published some years before 

their date, and why, 249 

various meridians from which to compute, . 249 

table of the lengths of the degrees of, . . 251 

Lunar variations, 184 

Mars, 131 

physical constitution of, 132 

Meridian line, how found, 74 

Mercury, 126 

physical constitution of, 127 

Masses of the sun and planets, table of the, 155 

how known, 165 

Milky-Way, 86 

Meton, cycle of, 283 

Medium resisting, 293 

Moon, new, • . 68 

the, 107 

physical constitution of, ...*.. Ill 
rays of the, have neither heating nor chymical 

properties, 114 

superstitions respecting, refuted, . . . 114 

the horizontal, 254 

" figure of, 258 

the Harvest and the Hunter's, 258 

the full, at the polar circles, 263 

" at the poles, 264 

Objects, apparent size of, modified by the supposed distance at 

which they are situated, 27 

Pallas, 135 

Parallax, annual, 96 

Planets, primary, 48 

secondary, 48 

names of, 49 

orbits of, 49 

distances of, &c 98 

inferiour, 126 

superiour, , , . . . 131 

telescopick, 134 

stationary and retrograde appearances of, illus- 
trated, 180 

Plants, a species of vegetable thermometer, .... 274 

Preface, 5 



INDEX. Xll 1 

Rays, luminous, sensation produced by, has a sensible dura- 
tion, 39 

temperature of, bow measured, .... 106 

Revolutions of the planets, table of tbe, 156 

Rotation, npon their axes, of tbe sun and planets, . . . 156 

Saturn, 141 

satellites of, ....... 142 

rings of, 143 

" appearance of in the different signs, . 144 

" measures of the, 145 

*' of new discoveries respecting. . . 147 

shape of, mistaken by Herschel, .... 149 

Satellites of Jupiter, table of the, 157 

Saturn, 158 

Uranus, 158 

Scintillation of the fixed stars explained, 72 

Stars, how designated in maps and upon globes, .... 47, 

uniform motion of ihe, 70 

positions of, how determined, 76 

fixed, of the, 81 

" motion detected in some of these, . . 81 

" number of greatly overrated, ... 82 

li constellations of the, names of, &c. . 83 

variable and new, . 90 

fixed, distance of, 95 

" recent discoveries respecting, ... 97 

" lost, 98 

Sun, physical constitution of, . 102 

spots on, • . . 103 

motion of, 68 

Teachers, note to, 16 

Telescopes, refracting, 29 

reflecting, ... 32 

front view, . • 34 

cross wires for, how made, 35 

magnificent one, 31 

large and perfect refracting ones unknown in 

England, 31 

Tides, of the, 231 

circumstances of the phenomena of the, . 241 
absence of, in the Baltick and Mediterranean ac- 
counted for, 242 

Time, sidereal, how found, 80 

equation of, 288 

Trade winds, 275 



XIV INDEX. 

Universal attraction, 160 

Uranus, 150 

errours in the history of the discovery of, cor- 
rected, 151 

satellites of, 153 

" peculiarities of the, 153 

Variable winds, 279 

Venus, ' 128 

physical constitution of, ...... 130 

transit of, 99 

Vesta, , 136 

Velocity of falling bodies at the suriace of the sun and planets, 

table of the, 156 

Vision, act of, how performed, 37 

distinct, at unequal distances, how produced, 

by the eye, .*...-... 37 

effect upon, by operation for cataract, . . 38 

Volumes of the sun and planets, table of the, .... 155 

Week, days of the, 286 

Year, the Egyptian and Perdan, 282 

the great Canicular, 282 

Zodiack, signs of the, 58 

' ' etymological explanation of, ... 59 



AUTHORITIES. 



The following are among the authorities which have been 
critically consulted during the preparation of this work: 

Lapif) 119 du .Monde; 

Pontecoulant, Theorie Analytique du Systeme du 

:ide; 
Delambre, l'Astronomie theorique et pratique, 
u Histoire de 1* Astronomic Ancienne, 

" " " du moyen age, 

" " M " moderne, 

** " " au dix-huitieme Siecle; 

Bailly, " " " ancienne, 

" »« •• moderne; 

ntucla, Histoire des Mathematiques; 
Pingre, Cometographie; 

Denon, Voyage dans la basse et la haute Egypte; 
Biot. Recherches sur FAstronomie Egyptienne; 

lonnier, Observations Astronomique; 
Humboldt. Voyage aux regions e;minoxiales du nou- 

veau continent; 
Struve, Dorpat Catalogue of Double and Multiple 

Je, Bibliograpbie Astronomique; 
["ems; 
- hebdomadaires des Seances de FAca- 
e.ie des Sciences; 
?phical Transactions of the Royal Society, 
i Ion; 

le American Academy; 
Transactions of the American Philosophical Soe 

. tronomy; 
iuction to Prac tronomy; 

Association for the Advance- 
mei. 



NOTE TO TEACHERS. 



The etymological explanation of the several names 
of the signs of the zodiuck has often been attempted; 
and there is no lack of theories and suggestions, in the 
books, upon their probable origin and signification. 
Reference has been made, for this purpose, to the 
fables of Castor and Pollax, the abduction of Europa, 
the Argonautick expedition, the passage of Venus and 
Cupid over the Euphrates — in short, to whatever my- 
thological fable could be found, in the prolifick cata- 
logue of these, that savoured of symbolical representa- 
tions which might serve as their explanations. 

The true meaning of these several names having 
finally been discovered, through the successful re- 
searches of the Institute of Egypt, at Paris, these are 
inserted, at page 59, and onward; but they differ so 
widely from what has hitherto been taught, upon this 
subject, that it has been deemed advisable to gke them 
in the several languages through which they have 
come to us, as well as in Englis h the better to show 
their authenticity. Another reason for this has been 
found in the highly figurative language employed, 
which often admits of only a very loose and general 
rendering in English, that could scarcely fail, if alone, 
to prove unsatisfactory to those whose business it is to 
communicate instruction. 

This explanation is made as an apology for the ap- 
pearance of Coptick, Arabick, Greek and Latin words 
and phrases, in an elementary book; but it is hoped 
all cause of objection is removed by the fact shown, 
namely, that these are in no sense intended for the 
exercises of the pupil, but only for the satisfaction of 
those to whom he is accustomed to look for elucida- 
tion. 



ASTRONOMY. 



CHAPTER FIRST. 

OP ASTRONOMICAL INSTRUMENTS. 

Before entering upon the domain of Astronomy, 
strictly speaking, it is important to know the instru- 
ments which opticks has placed at the service of this 
science, and to the aid of which it owes so much of its 
perfection: instruments of which the power has aug- 
mented, the sphere of activity of our organs so as, in 
some degree, to bring the mechanism of the universe 
within our view. The study of these instruments will 
form the object of this chapter. 

The construction of reflecting telescopes depends 
upon the laws of the reflection of light, and that of 
refracting telescopes upon those of its refraction. Let 
us first examine these two important properties of the 
luminous fluid. 

GENERAL LAWS OF THE REFLECTION OF LIGHT. 

If we cause to fall, obliquely, a pencil of solar rays 
upon a polished surface, we may observe the following 
phenomena: 

1st. A part of the luminous rays are reflected in a 
certain direction, and if we place the eye in the line of 
2 



18 OP ASTRONOMICAL INSTRUMENTS. 

this direction, wc see an image of the sun, in the pro- 
longation of the reflected rays: 

2d. The point where the incident, or approaching 
rays encounter the polished surface is visible in all 
directions; but it appears incomparably less luminous, 
from any other point of view than when observed in 
the line of the reflected rays — the only view which 
gives a regular image of the sun: 

3d. A portion of the incident light is not reflected, 
but passes, according to known laws, through the 
reflecting body, if it is transparent; while if it is 
opaque the same quantity of light is absorbed. 

Here, then, we have three distinct phenomena: 
one portion of the incident light is reflected regularly, 
in a particular direction; another part is reflected in- 
differently, in all directions, and disseminated as if by 
a body not polished, while the remainder either passes 
through the reflecting body, or is absorbed by it. 

But what is the direction followed by that portion of 
light which is regularly reflected? We find: 

1st. That the incident and reflected rays are compri- 
sed in the same plane, perpendicular to the reflecting 
surface; and, 

2d. That the incident and reflected rays always form 
equal angles with the reflecting surface; or, that the 
angle of reflection and the angle of incidence are equal. 

Such are the two general laws of reflection; and 
they very readily explain to us the formation of ima- 
ges, in this way. 

Take, first, the plane mirror; suppose, fig. 1, S the 
radiant point, O the eye of the observer, and A B 
the plane reflector. Among all the luminous rays 
emanating From S, there will be one of them, as S I, 
which, after being reflected, by the mirror, will meet 
the eye at O, in the direction I 0, thus making the 
angle of incidence equal to the angle of reflection. If, 
from the radiant point S, we draw the perpendicular 



OP ASTRONOMICAL INSTRUMENTS. 



19 




S A, which encounters, at A, the reflecting surface; 
and if we prolong this perpendicular, upon the oppo- 
site side of the mirror, 
by the quantity A D, 
equal to S A : then, from 
the point D, draw the line 
D O, to the eye; DO 
will be the direction of 
the reflected ray, and the 
B point I, where it cuts the 
surface of the mirror, 
will be the point of inci- 
dence. Moreover, if the 
luminous object and the 
eye are both supposed to 
be mathematical points, 
without sensible extent, the ray determined by the pre 
ceding rule, is the only one which can be rejected to 
the eye. 

But the opening of 
the pupil, which ad- 
<i mits rays into the eye, 
is not a mathematical 
point; it is a space 
which, in man, has 
about ,07874 of an 
inch diameter, and 
which we may repre- 
sent by L L, fig. 2. 
All the reflected rays 
which can enter this 
opening, will come, 
then, to the retina, 
and thus contribute to 
vision. Each ray 
being governed by 
the same law, which we have here explained, it is 




20 



OP ASTRONOMICAL INSTRUMENTS. 



evident that these would form a cone with a circular 
base, of which the apex would be D, and the base 
L L. To the eye penetrated by reflected rays the 
luminous body appears placed at the point from which 
the visible rays seem to diverge. Thus, the eye being 
placed at 0, the luminous point, seen by reflection, 
would appear at D: that is to say, the same distance 
behind, that it really is before the mirror. 

If the radiant object has a defined extent, each of its 
points of luminous surface will contribute to the for- 
mation of its image, according to the rule which we 
will explain; and the sum of these partiaj images con- 
stitutes that of the entire object To illustrate this, sup- 
pose the object to be an arrow, S S', fig. 3: the image 

of the base S, of the 
arrow, will be shown 
a: D: the poin:. S . 
in like manner at D ; 
and the intermediate 
portions of the arrow 
will be seen, in the 
rightline,DD'. Thus 
the entire ima g 
the arrow will be com- 
prised between D O, 
DO: its actual size, 
D D . will be equal to 
S S'; in other words, 
to that of the object 
itself: but it will ap- 
pear reversed, from 
right to left 

The preceding ex- 
planations will suffice 
for resolving all questions which may arise, relating 
to the reflection of light and the vision of objects by 
plane mirrors. 




A 



OP ASTRONOMICAL INSTRUMENTS. 21 

As to curved surfaces, whatever may otherwise be 
their figure, for determining, in general, the apparent 
place, the form and the magnitude of the images which 
they reflect, it will suffice to conceive the reflection 
of each luminous ray to be made upon the plane tan- 
gent to the surface, at the point of incidence. Yet in 
practical use it is unnecessary to ascend to this gene- 
ral law, since we can never employ any other than 
spherical, concave or convex mirrors, these being the 
only ones that we can construct and polish, with accu- 
racy; and even in these, to obtain the most perfect 
images, it is necessary that the luminous rays fall very 
near perpendicularly upon their surface. We shall 
therefore confine ourselves, here, to these figures, 
alone. 

Suppose, then, a luminous point, in space, sends 
forth its rays upon the different parts of a spherical 
surface, either concave or convex, and selecting one 
of these rays, let us seek the direction in which it will 
be reflected. 

pig. 4. Suppose M A M', fig. 4, 

the spherical mirror, S the 
luminous point, and S I, 

- the incident ray under con- 

j^><! * sideration. From the point 

R c I to the centre of the sphere 

draw the line I C; then take 
the angle C I R, equal to C 
I S, and I R will be the di- 
rection of the reflected ray. 

By proceeding in like manner with all the incident 
rays emanating from S, we shall find, either by dia- 
gram or by calculation, that the reflected rays all fall 
very nearly together, within a small space denomina- 
ted the focus, where they form, by their concentration, 
an image of the point S. Experiment has confirmed 
the correctness of this theory. 
2* 



\ 



22 OF ASTRONOMICAL INSTRUMENTS. 

Analogous reasoning and construction show us that 
the image produced by a convex mirror is always 
ideal, and is formed behind the mirror, so that, although 
one may view it, by direct vision, it cannot be exhibi- 
ted upon ground glass, or a screen. 

GENERAL LAWS OP THE REFRACTION OF LIGHT. 

We have seen in what manner luminous rays are 
influenced when reflected from the surface of bodies; 
and we are now to investigate the action to which 
those are subject, which traverse the substance of the 
bodies upon which they fall. 

These last, when the angle of incidence is oblique, 
continue not in a right line, but are made to deviate 
from that direction; and it is this phenomenon that is 
called the refraction of liglvt. 

Whenever a luminous ray passes obliquely from one 
medium into another, it is refracted; and the extent of 
this refraction, or deviation from a right line, depends 
upon the difference which exists between the densities 
and the natures of the two mediums. In all bodies not 
crystallized the refracted ray is simple, and follows the 
prolongation of the plane of incidence. It deviates 
more or less from a right line to the common surface 
of the two mediums according as it passes from the 
more rare to the more dense, or from the more dense 
to the more rare of them. 

It remains to determine the relation which exists, 
for each incidence, between the obliquity of the inci- 
dent rays to the perpendicular, and that of the refracted 
rays, in order to be able to calculate one of its direc- 
tions, the other being known. 

This brings us to the two following laws, discovered 
by Descartes: 

1st The incident and refracted rays are always com- 
prised in the same plane, perpendicular to the common 
surface of the two mediums: 



OF ASTRONOMICAL INSTRUMENTS. 23 

2d. The sine of the angle of refraction is to the sine 
of the angle of incidence in a constant ratio, under all 
incidences, for the same medium. This is denominated 
the ratio of refraction. 

Refraction is always accompanied by a remarkable 
phenomenon. The refracted ray is decomposed, and 
separated into rays of different colours, of which the 
refrangibility increases from the red ray, where it is 
the least, to the violet, where it is greatest. This phe- 
nomenon is designated, the dispersion of light. 

In addition to the seven prismatick colours, experi- 
ment has shown, in the refracted ray, calorifick or 
heating rays, of which the intensity, according to the 
prismatick arrangement of the colours, augments from 
the violet to a point beyond the red; and chymical rays, 
of which the intensity augments in the opposite direc- 
tion; that is, it is greatest in the red ray, and least at 
a point beyond the violet. 

OF LENSES. 

When a luminous ray is received upon a prism of 
glass it is refracted and caused to approach nearer the 
base of the prism; conforming also, in other respects, 
to the laws we are about to explain. Now it is easy 
to conceive a system, an assemblage of prisms, cut 
and disposed in such a manner that the rays refracted 
by them would tend to one and the same point We 
realize, at once, how important it would be to be able 
thus to concentrate a great number of luminous rays; 
but the difficulty of constructing, with sufficient pre- 
cision, an apparatus of this kind would have opposed 
great obstacles to the progress of science, if, by unex- 
pected good fortune, it had not been discovered that 
the necessary principles were all combined in the sphe- 
rical lens, which, indeed, is no other than an assem- 
blage of prisms, and which may be constructed with 
exactness and comparative facility. 



24 



OP ASTRONOMICAL INSTRUMENTS. 



Of these we distinguish several kinds: 
1st. Double convex lens, or glass, fig. 5. 

Fig. 5. The resemblance, in form, of this 

<r -~- --^^ glass to the seed of the lentil origi- 

^^- — — -^'nally gave it the name of lens, but 

this name has since been indiscriminately applied to 
all spherical glasses.* Fig. 6. 

2d. Plano-convex, fig. 6. 
3d. Concavo-convex, or meniscus, 
fig. 7, and fig. 8. 

Fig. 7. Fig. 8. 




Fig. 9. 



4th. Plano-concave, fig. 9. 

5th. Double-concave, fig. 10. ( 

Fig. 10. L^~- —^ 

All these different forms of glasses 
may be ranked in two classes, ac- 
cording as the base or point of the prism is turned to- 
wards the axis of the lens; and as refraction always 
takes place in a direction towards the base of the prism, 
the former will cause to converge, and the latter to 
diverge the luminous rays which fall in a parallel man- 
ner upon their surfaces: hence we sometimes designate 
one class convergent, and the other divergent lenses, 
or glasses. 

It is easy to see how such glasses would assist 
vision, by correcting the too strong or too feeble con- 
vergence of rays for the eyes of persons that are de- 
fectively long or short sighted; but our object does not 
require us to dwell upon this point 



* Lens, Latin; Lentille, French; Lentil, English; a kind of 
plant bearing a pod in which are contained seeds resembling, in 
shape, the double convex lens, fig. 5. 



OP ASTRONOMICAL INSTRUMENTS. 



25 



Fig 11. 

ssssssss 




Suppose an assemblage of parallel rays to fall upon 
a convex lens, that we may examine more closely this 
phenomenon. Among the incident rays, S, fig. 11, 
there is one which coincides with the 
axis of the lens, and therefore traver- 
ses it without refraction. But with the 
remaining rays this is not so. They all 
experience a refraction; and this is 
greater the farther the point at which 
they enter the lens is situated from its 
axis; so that the whole, after passing 
through the glass, are made to converge 
to the point F. This point is named 
the focus of the lens. It is evident that 
the greater the convexity of the lens 
the stronger will be the refraction, and 
consequently the nearer will be the focus of the lens 
to itself. On the other hand, if the luminous rays, 
after having been assembled at the focus F, are radia- 
ted back, to the lens, upon the same angle, they will, 
after passing through the lens, issue from the opposite 
surface of it in lines parallel to each other: from which 
it follows that luminous rays, radiating from any body 
placed in the focus of such a lens, and so directed as 
to fall upon all parts of its surface, would experience 
the same refraction, and issue, after traversing it, in 
the same parallel manner. 

This property of the lens has given rise to a very 
useful apparatus, that serves as the base for the con- 
struction of the lights of lighthouses; which is no 
other than an assemblage of four lenses, in the com- 
mon focus of which is placed a lamp. The luminous 
divergent rays which escape from the lamp traverse 
the lenses and issue, in parallel beams, from their op- 
posite surfaces; they consequently are not enfeebled by 
dispersion; nor do they lose any of their intensity ex- 
cept that portion which is absorbed by the imperfect 



26 OF ASTRONOMICAL INSTRUMENTS. 

transparency of the atmosphere: hence they remain 
sufficiently brilliant to be clearly visible even at the 
horizon. But as the diameter of these collections of 
parallel rays is necessarily circumscribed, their light 
can only be shed upon a portion of surrounding space, 
at any one moment. To remedy this inconvenience 
the whole illuminating apparatus is so adjusted as, by 
means of clockwork, to revolve, entire, upon a com- 
mon axis, in a given time. The period of rotation of 
each being different from every other, and that of each 
particular one being previously known to navigators, 
and any one of these times of rotation being easily 
determined from on board a vessel, by the respective 
durations of the dark and luminous intervals caused 
by the rotation of the lantern, such rotation not only 
enables the mariner to distinguish lighthouse lights 
from any other on shore, but also to determine what 
particular one is in view; and thus to instruct him upon 
what coast he has arrived, and even the particular 
part of that coast. 

Another property of these lenses is that of magnify- 
ing the images of objects. Let it be remembered that 
the apparent dimensions of a body depend upon the 
angle under which it is seen, and that this angle varies 
in the inverse ratio of the distance of the object from 
the eye of the observer. From this it follows that an 
object would appear of enlarged dimensions by simply 
placing it near the eye, if the organs of vision were 
capable of acting, at that point, in perfection; but the 
divergence of the rays, under these circumstances, 
renders the image confused. To remedy this we ob- 
serve the object through a convergent lens. The 
parallelism of the rays, in this case, admits of the 
near approach of the eye, and the image of the object 
appears under an angle equal to that under which the 
object would appear, to the naked eye, if the organs 
of vision could act directly upon it, at so small a dis- 



OP ASTRONOMICAL INSTRUMENTS. 27 

tance. It is evident from this that the magnifying 
power of the lens is in proportion to its focal length, 
and that such power is increased as the focal length 
is diminished. 

In the experiment of which we have spoken the idea 
which we form of the real magnitude of the object is 
determined by the angle under which it is seen, with- 
out our being able to modify it by any previous expe- 
rience upon the relations of the distance with the visual 
angles. But in all ordinary acts of vision with the 
naked eye, this is not so; for in the opinion we there 
form of the magnitude of objects we always have re- 
gard to these two conditions, namely, the angle under 
which we view the object, and the actual distance at 
which we suppose that object to be situated. It is in 
this way that we are enabled to judge as accurately as 
we do of the stature of two men placed at unequal dis- 
tances from us, and of course seen under different an- 
gles; because the effect of distance, in diminishing 
objects, is remembered and appreciated. So true is 
this that we uniformly fall into errour in judging of the 
real dimensions of objects, if we are much deceived in 
the distance at which we view them. It is for this 
reason that objects which we observe through convex 
spectacles do not seem to us to be magnified, because 
we suppose them brought nearer, while in fact the 
glasses magnify some two or three times, perhaps, as 
we may become convinced by observing the object 
with both eyes at the same time, while one is aided by 
the glass and the other is not. We may farther illus- 
trate this by the following simple experiment: place 
an object upon a horizontal plane, and range the eye 
in the prolongation of that plane; then, looking at the 
object, press lightly with the finger, the inferiour eye- 
lid, in such a manner as to produce two images, when 
that which is nearest will appear smaller than the 
other, and will seem to diminish in proportion as it 



28 



OF ASTRONOMICAL INSTRUMENTS. 



approaches more. A full proof that the supposed dis- 
tance of the respective images determines our judgement 
respecting their magnitude is that they will appear to 
us of equal size when we shall place the object in a 
vertical plane, in such a manner as to obtain these two 
images one above the other. 

But, to return to the lenses. We have seen by 
what laws a beam of parallel rays is refracted. We 
will now examine the manner in which rays emana- 
ting from different points of an object are refracted. 
Suppose A B, fig. 12, an enlightened object. It is 
Fig-. 12. evident that from each of the extre- 
mities of this object there will depart 
a pencil of rays, of which the point 
of convergence will be found some- 
where upon the prolongation of the 
direction of the central ray of that 
pencil which, having encountered 
that part of the lens that presented to 
it parallel surfaces, had not been sub- 
jected to refraction. The point A, 
then, in the figure, would be delinea- 
ted at A', the point B, at B', and the 
intermediate points upon the right 
line which joins A' and B'; and if 
these rays be received upon a sheet 
of paper, or upon ground glass, a reversed image of 
the object A B will there be exhibited. 

We have seen, above, that a ray of solar light, re- 
fracted, is decomposed in rays of different colours. 
Such decomposition, therefore, colours the images of 
objects and renders them confused. This evil is so 
serious a one that Sir Isaac Newton long sought to 
remedy it, and finally, despairing of doing so, he pro- 
claimed the attempt a desperate one, and that this 
must cut off all hope of constructing improved refrac- 
ting telescopes. Happily the means have since been 




OF ASTRONOMICAL INSTRUMENTS. 29 

discovered of remedying this difficulty. The remedy 
consists in employing lenses of substances which, 
while they disperse light equally, refract it unequally. 
The crown glass and flint glass of opticians, are such 
substances; and it is by combining, in proper propor- 
tions, lenses of these two species of glass that have 
been constructed the acromatick object-glasses of tele- 
scopes employed in astronomy at the present day. 

OF REFRACTING AND REFLECTING TELESCOPES. 

The astronomical refracting telescope, may be con- 
sidered as essentially composed of two glasses. One, 
which is named the object glass, receives the luminous 
rays from the object, and of these forms an image of 
that object, in its focus; the other, which is designated 
the eye glass, is placed near the eye, and assists vision 
in examining that image. The magnifying power of 
this species of telescope arises from two causes: the 
image formed in the focus of the object glass is already 
magnified, when viewed with the naked eye, because 
the observer places his eye only some seven or eight 
inches from it, being a distance much less than that 
which separates the lens from the focus, and it is con- 
sequently seen, thus, under a much greater angle; but 
the magnifying power of the instrument is principally 
derived from the eye glass, which is a convex lens 
whose focal distance is very short. These telescopes 
are sometimes very powerful: they have been so con- 
structed as to magnify the object, viewed through them, 
one thousand times. 

The cut, fig. 13, will give an idea of this kind of 
telescope, with one of the numerous methods of mount- 
ing it for use. In this figure the tube, a b, and the 
glasses it contains, constitute the telescope. The ob- 
ject glass is at b. This is directed towards the celes 
tial body to be examined, and the rays which fall upon 
3 



80 OF ASTRONOMICAL INSTRUMENTS. 

Fig. 13. 




it are refracted to a focus within the tube, near cu 
The image formed at this focus, is viewed through 
the eye piece a, of which each telescope usually has 
several, of different magnifying powers. The finder 
is a small tube, with glasses, and having two wires 
crossing each other at right angles, in their focus. 
When the star is brought, by observation through this 
finder, exactly upon the crossings of the wires, ;it 
is then in the proper position to be viewed through 



OP ASTRONOMICAL INSTRUMENTS. 81 

tho main tube. This finder is usually fastened upon 
the .'3ide of the large tube : in this cut it is shown at d. 
By turning the handle g, a rotary motion is given to 
the telescope, upon the upright pillar which supports 
it; and an apparatus equally simple, but not shown 
here r gives a vertical motion, at pleasure. With these 
two motions the instrument is readily pointed in any 
desired direction. 

Instruments of this construction, of large size and 
high degrees of perfection, have ever been entirely 
unknown in England; and have been exclusively con- 
fined to the continent, both in their fabrication and 
their use. 

The most perfect and magnificent telescope of this, 
or, i ndeed, of any kind, ever constructed — unless a 
new one which has recently been erected at Rome shall 
prove> its superiour — is that manufactured by the cele- 
brated Fraunhofer, of Munich, for the Imperial Obser- 
vatory, at Dorpat, in Russia, which is under the direc- 
tion of Professor Struve. It is the property of the 
Emperour of Russia, and was erected in 1825. Its 
focaL length is 14 feet, and the diameter of its object 
glass is a fraction less than 9 J inches. Clockwork is 
so aj )plied to this telescope as to give it a regular and 
perfi ict siderial motion, so as to keep a star constantly 
in tbie centre of the field of view, thus producing the 
appe;arance of a state of entire rest, in the starry 
sphere; and this motion can be varied at pleasure, to 
that of the sun, or the moon. 

This instrument, in the hands of Professor Struve, 
has already given vast aid to the science of astronomy; 
particularly through his examinations of double and 
mul tiple stars, nebulae, &c, the results of which fur- 
nish some of the most exciting phenomena that modern 
research has produced. 

The reflecting telescope is constructed of a polished, 
metallick mirror, in the focus of which the image is 



32 



OP ASTRONOMICAL INSTRUMENTS. 



formed, by reflection. Of this instrument there are 
several forms, differing widely from each other, in 
many particulars of their construction, and generally 
distinguished, among astronomers, by the names of 
the contrivers of the several peculiarities, respectively. 
Of these we will endeavour to convey a general idea, 
by drawings and descriptions; commencing with the 
Gregorian Reflector, fig. 14. 

Fig. 14. 







In this figure A B is a concave, metallick mirror, 
with a hole through its centre. In front of this mir- 
ror, is placed a small, concave one, C D, which is 
moveable by the screw W. An eye piece, consisting 
of two convex lenses, E, F, is placed immediately be- 
hind the great mirror. The incident rays M A, N B, 
falling upon the great mirror A B, are by it reflected 
back to the small one C D, and by this last they are 
again reflected, and passing through the hole in the 
centre of the great mirror, they reach the eye of the 
observer, after traversing the two convex lenses, E, F. 
The rays first reflected by the great mirror, A B, will 
form an inverted image of the object from which they 
emanate, at m n; and this image, by the subsequent 
reflection of the mirror C D, will be again formed,, 
but in an erect position, at m' n', between the convex 
lenses E, F. 

The Cassegrainian Reflector differs from the Gre- 
gorian only in having its small mirror convex instead 
of concave; by which the length of the instrument is 



OF ASTRONOMICAL INSTRUMENTS. 33 

much reduced; and it is generally admitted that this 
form gives more light and exhibits an image better de- 
fined, by reason of the correction, by the convex mir- 
ror, of the aberration of the concave one. 

The Newtonian Reflector is represented by figure 
15; where A B is a concave mirror, and m n the in- 
Fig. 15. 



verted image which it forms of the object from which 
the rays M, N proceed. As the eye cannot be intro- 
duced into the tube, to view this image without ob- 
structing the light falling upon the mirror from the 
object to be examined, a plane mirror, C D, is placed, 
upon a proper support, so as to receive the cone of 
reflected rays, between the great mirror and the image 
m ft, and being inclined 45° to the axis of the great 
mirror, it reflects the image from that mirror to m 1 n', 
where it is magnified by the eye glass E. To avoid 
the loss of light by a second reflection Newton pro- 
posed, in place of the small, plane mirror, C D, to 
substitute a rectangular prism, in which the light 
would be wholly refracted; but as such prisms, suffi- 
ciently perfect, both in material and mechanical exe- 
cution, can rarely if ever be produced, these telescopes 
are not in use. 

But the form of this species of telescope which is 
now most employed, for astronomical purposes, is the 
Front View Reflector, figure 16; where A B C D is 
a tube, at the bottom of which is the reflecting, con- 
cave mirror p. This mirror is so inclined that the 
image of the object, from the reflection of the incident 
3* 



M 



OF ASTRONOMICAL INSTRUMENTS. 

Fig. 10. 




m y s fi Si w iH be formed in the focus of the mirror 
near H. The image is thus thrown to the side of the 
tube that the observer may not shut out light from the 
mirror. In using this telescope the observer, at H, 
sits with his hack to the object, and views the image, 
through an eye glass at the mouth of the tube, whose 
length varies but little from the focal distance of the 
reflector, and whose end, A H, must be directed to the 
object, that the incident rays may enter the tube to fall y 
upon the mirror. 

For the convenience of readily giving this instru- 
ment its proper direction towards the object to be 
viewed, a finder, I, of the usual construction, may be 
attached to the curved arm K, which, rising from the 
side of the tube near D, is shown to pass round the 
observer, H, and sustains the finder behind him, in a 
line parallel to the axis of the tube. To use this finder 
the observer has only to face about, and, with his hand 
upon the apparatus for moving the tube, regard the 
object, while he varies the direction of his telescope 
at pleasure. 

As the best polished surfaces do not reflect but about 
one half of the light which falls upon them, there is a 
manifest disadvantage, in regard to light, in the use of 
all the forms of this species of telescope. We may 
magnify the image, at pleasure, by means of eye 



OP ASTRONOIVIICAL INSTRUMENTS. - &5 

glasses, so long as the light of which it is formed is 
sufficiently strong to render it distinct: and it is mani- 
fest, therefore, that a reflecting telescope, with two 
mirrors, can have but about one fourth the power of a 
refracting telescope of the same diameter; while those 
with only a single mirror, all else being equal, have 
about one half the power of such refractor. It is for 
this reason that the front view reflector is preferred 
over either of the other forms. 

For measuring the heights of the various heavenly 
bodies, and for a variety of other purposes, telescopes 
have metallick threads variously adjusted in their fields. 
These threads exceed, in fineness, the threads of the 
spider's web, and the process of their manufacture is 
equally curious and ingenious. They are of platina; 
and the metal is first reduced, in the ordinary way, to as 
fine a wire as possible. These wires are then placed 
in cylinders and melted silver is cast around them, so 
that the platina constitutes the axis of the cylindrical 
mass. In this condition they are passed, in succession, 
through all the wire plates, even to the very finest. 
Of course, in this process the platina, as well as the 
silver, is continually reduced in size; and when the 
operation has been completed, the wire is immersed in 
nitrick acid, which dissolves the silver, forming the 
outside of the wire, but leaves the fine thread of pla- 
tina, in the centre, without acting upon it. 

CONFORMATION OP THE EYE. 

;t 

We will terminate this chapter with an examination 
of the organ of vision; the most wonderful of all opti- 
cal instruments. In man this organ is formed of seve- 
ral different transparent mediums, the curves and the 
refractive powers of which are so combined as to cor- 
rect the aberrations of sphericity and refrangibility. 
The images are formed upon a nervous membrane, at 



36 OF ASTRONOMICAL INSTRUMENTS. 

the bottom or back portion of the eye, which trans- 
mits to the brain the sensations it receives. 

This organ is composed of three mediums, differing 
in form and in refractive powers. The first is a me- 
niscus, filled with a transparent liquid, in appearance 
resembling water, and which, for this reason, has re- 
ceived the name of aqueous humour. To this succeeds 
a solid, transparent body, nearly of the form of a 
double convex lens, its front surface less curved than 
its rear one; and having its curvature continually di- 
minished by the approach of old age. This substance 
is designated the crystalline humour. The posterior 
cavity is supplied with a viscous liquid, resembling 
melted glass, and which, on this account has been 
named vitreous humour. The envelope which contains 
all this system, may be considered as formed by the 
prolongation and extension of the teguments of the 
optick nerve. The exteriour tegument, constituting 
the envelope, is hard and opaque, yet flexible, like the 
cornea, and is called the sclerotick coat, or opaque 
cornea. Directly in the front portion of the eye this 
membrane becomes thinner, and is there transparent, 
like a lens, that the light may pass through it: this is 
called the transparent cornea. It is exceedingly tough, 
of equal thickness throughout, and is capable of op- 
posing great resistance to external violence. The 
choroid membrane is a delicate membrane lining the 
inner surface of the transparent cornea, and is coated, 
upon the inner side, with a black pigment. This black 
coating suggested to us the practice of painting black 
the inside of the tubes of our telescopes: in both cases 
the black surface absorbs the light that falls upon it, 
and thus prevents that confusion which would result 
from the multiplied reflection of scattered rays. The 
interiour and medullary portion of the optick nerve 
forms a nervous membrane of a grayish white colour, 
lying immediately within the black coating of the 



OP ASTRONOMICAL INSTRUMENTS. 87 

choroid membrane, and is known as the retina. It is 
upon the surface of this membrane that the images of 
objects are supposed to be painted. 

It is now easy to sec how the act of vision is per- 
formed. The rays emanating from exteriour objects 
fall upon the transparent cornea, traverse, success- 
ively, the aqueous, crystalline, and vitreous humours, 
and fall, concentrated, upon the retina, at the focus of 
the instrument, where they form a small, inverted 
image. This result may be verified upon the eyes of 
men or animals, if extracted immediately after death. 
If the sclerotick coat be reduced in thickness, at its 
superiour part, and a luminous object be placed at a 
convenient distance before the eye, the observer may 
see, from behind, a well formed and clearly denned 
image of the object, at the bottom of the eye, which 
will vary in the inverse ratio of the distance. In op- 
tical instruments vision is rendered equally perfect at 
different distances, by varying, proportionately, the 
focal length of the instrument. By what mechanism 
is this condition fulfilled, in the eye, where vision is 
equally distinct at very various distances'? That some 
adjustment of the eye, analogous to the variations of 
the focal distances in the telescope, takes place, is 
proved by the fact that a certain time, and even a cer- 
tain effort of the eye is necessary to adjust its powers 
to a change in the range of vision. This may readily 
be demonstrated by placing some small object, a hair, 
for instance, at a little distance from the eye, in such 
a manner that its image is projected upon another ob- 
ject, more distant; when it will be found impossible to 
see both objects, with distinctness, at the same time; 
but the eye must be made to pass from one to the other, 
in succession, to obtain perfect views of each. But, 
though the fact is well established, yet anatomy has 
essayed in vain to discover by what mechanism this 
organ is enabled thus to vary its effects. It was at 



38 OP ASTRONOMICAL INSTRUMENTS. 

one time supposed that the anterior part of the cornea 
assumed, at pleasure, a greater or less degree of con- 
vexity; and, again, that the retina was endowed with 
a small degree of mobility, and that in like manner it 
receded or approached, so as to follow the displace- 
ments of the focus; but the most precise experiments 
have demonstrated the falsity of these two hypotheses. 
There remains, then, the crystalline humour, to pro- 
duce this phenomenon; and we think, notwithstanding 
that it is impossible for us wholly to reconcile this 
view with the data furnished by anatomy, that it is to 
this humour that the eye owes the power in ques- 
tion; for in cases of injury to this humour the eye loses 
the faculty of distinct vision, at unequal distances. 
This is the case with persons who have submitted to 
an operation for cataract, (which operation consists in 
rupturing this humour, and removing from the line of 
vision that portion of it which disease has rendered 
opaque,) for such persons only see well, afterwards, 
at a given distance, and this an increased one; consti- 
tuting them what are sometimes designated long sighted 
persons. 

But how does the act of vision give rise to sensa- 
tion? This we know not: we only know that the im- 
pression produced upon the retina is transmitted to the 
brain by the optick nerve. From this data Mariotte 
supposed that the nearer the image approaches to the 
point where the nerve begins to spread or open itself 
upon the retina, the more vivid the sensation would be, 
and that it attained its maximum of intensity when it 
was formed upon the very point of the nerve's termi- 
nation. But experience yielded him a diametrically 
opposite result; for he saw, by a very simple experi- 
ment, that this part of the retina is insensible, and 
that an object becomes invisible the moment it is pla- 
ced in such a manner as to cause its image to fall upon 
that point. 



OP ASTRONOMICAL INSTRUMENTS. 8^ 

The axis of the eye, that is the direction in which 
we habitually regard objects, is not that in which wo 
see objects best. The part of the retina which corres- 
ponds to this axis is hardened by use, and is less sen- 
sible than the adjacent parts. We consequently see an 
object much better when regarding it a little upon one 
side of the axis of the eye. This is the reason why 
astronomers say that, to view a star well, it is neces- 
sary to look a little upon one side of the body, and not 
directly towards it. 

The sensation produced upon the retina, by lumi- 
nous rays, has a sensible duration. This may be de- 
monstrated by moving a burning coal, rapidly through 
a circle, in the air, when the entire circle thus descri- 
bed, will appear luminous and glowing. The same 
may be shown by placing a burning coal upon a wheel 
revolving rapidly, and so adjusted that it shall pass, at 
each revolution, before a small hole in a screen, through 
which it is visible only when directly opposite. In 
such a case the coal will appear to be constantly pre- 
sent, and at rest, if its revolution is so rapid as to 
bring it opposite the hole in the screen, twice in every 
second. 

If we look long upon the same colour, we thereby 
produce a morbid sensation in the fibres of the retina, 
which renders it less fit, for some time, to perceive 
that colour, and causes the complementary colours to 
prevail. It is for this reason that, after having stea- 
dily looked upon red or green, we see upon the objects 
we next observe spots of these colours, for they are 
complementary of each other; that is, by adding them 
together, white may be produced. 

It has been deemed probable that the fibres which 
perceive one colour are not the same as those em- 
ployed in distinguishing another. This, at least, 
seems to result from one incontestible fact, namely, 
that there are persons who are unable to perceive and 



40 HISTORY OF ASTRONOMY. 

distinguish all the shades of colours. Colardeau was 
one of these. He sometimes occupied himself in 
painting; and he one day coloured the background of 
a picture scarlet instead of sombre. When a friend 
pointed out to him his errour, he was unable to distin- 
guish any difference in the two colours. There is 
now living, in England, a celebrated scholar who dis- 
covered, while examining certain plants, that he was 
not conscious of all the colours; and the annals of the 
Academy mention an entire family who so confounded 
green with red that they could distinguish the leaves 
and fruit of the cherry tree only by their respective 
forms. Cases of this kind may be, and indeed are, 
continually found, when sought for, in almost every 
considerable neighbourhood; and Phrenology has- 
shown us that this fact, hitherto inexplicable, is insep- 
arably connected with defective developement of a cer- 
tain portion of the brain. 



CHAPTER SECOND. 

HISTORY OF ASTRONOMY DEFINITIONS. 

A thick cloud obscures the cradle of all the scien- 
ces; but that one of which the history is enveloped in 
the most profound obscurity, perhaps, of them all, is 
Astronomy. Equally ancient as the world, and inti- 
mately related to the first necessities of man, from his 
earliest existence, it cannot have failed to excite his 
curiosity and attract his observation. But these first 
elements of astronomy, collected in various places, 
greatly elongated from each other, were then lost to 
the science, as they now are to its history. 

We propose not, therefore, to detail the history of 
this science, from its infancy to the present moment, 



HISTORY OF ASTRONOMY. 41 

without for one instant losing sight of it, amidst the 
darkness by which it is surrounded, but only to catch 
a glimpse at it, here and there, through the cloud by 
which it is covered. 

The Chaldeans were perhaps among the first who 
occupied themselves with astronomy. This pastoral 
people inhabited the delicious regions of Asia, the 
finest country of the earth. Their habits of passing 
their nights in the open air, the serenity of their sky, 
the great extent of their horizon, must all have com- 
bined, very early, to induce these people to follow the 
movements of the heavenly bodies, and to study their 
imposing phenomena. 

From Chaldea astronomy rapidly spread itself into 
Egypt, that cradle of the arts and sciences, where it 
soon made great progress. The priests seizing upon 
it, engrafted it upon their system of religion, and made 
of it an instrument of domination over a credulous 
people, whom it was their study to retain in ignorance 
and superstition. 

The Phenecians appear to be the first who applied 
astronomical observations to the purposes of naviga- 
tion. They had observed that amidst the general 
movement of the sphere, one of the stars of the con- 
stellation Little Bear appeared ever to remain in the 
same situation. It was by this star that they guided 
their vessels; and such was their superiority that, in 
the time of Necho, at an epoch, when the other na- 
tions scarcely dared to quit the coast, they, having 
departed from the Red Sea, had made the tour of Af- 
rica, and returned, the third year, to the mouth of the 
Nile. 

About the same epoch astronomy was carried from 
Phenecia, by Thales, into Greece. Pie instructed the 
Greeks, who had hitherto regarded only Ursa Major, 
in the superiority of the pole star, as a guide for nav- 
igation. He taught them the laws of motion of the 
4 



40 HISTORY OF ASTROXOMY 

eun, and of those of the moon; from which he drew 
the explanation of the Ice s and the deter- 

mination of the solar year. w the causes of 

eclipses: and, it would seem, even the means by which 
to foretell them, since he acquired a great degree of 
celebrity for having announced od took place 

on the day of a battle between the Medes and the Lyd- 
ians. 

Anaximander, one of his di- vented the ter- 

J globe, and caused to be constructed, at Sparta, 
the gnomon : d him to observe the equinox- 

es and the solstices, and to determine, with much pre- 
cision, the obliquity of the ecliptick. The Greeks soon 
applied these new ideas to profit, in the purposes of 
navigation; but they were not grateful to him whose 
wisdom had taught them these truths. They proscrib- 
ed him, and would even have put him to death if Peri- 
cles had not checked the fur}" of the superstitious peo- 
ple. His crime consisted in having professed that the 
universe is governed by immutable la 

who lived about five centuries before 
our era, greatly conduced to the advancement of this 
science. He enriched it with almost all the great out- 
lines upon which it now repose : It T, as he who dis- 
:..; :; the world :: which Copernicus 
has left his name. It was he who first entertained the 
daring conception that the planets are inhabited globes, 
like that upon which we tread: and that the stars which 
people the immensity :: space are so many suns, des- 
tined to dispense heat and light to the planatary sys- 
tems which gravitate towards the:::. He saw. also, in 
comets, not transient and fugitive meteors, formed in 
our atmosphere, but permanent heavenly bodies, re- 
volving round the sun, in conformity with laws pecu- 
liar to themselves. 

The first who learned to class the climates according 
to the length 01 cavs and nights, was Pvtheas, who 



HISTORY OF ASTRONOMY. 43 

witnessed, if he did not cause the rise, among the 
Greeks, of a decisive taste for astronomy. Unable to 
satisfy themselves at Athens, these astronomers as- 
cended to the sources of this science: they went to 
study in Egypt; and Eudoxus brought back with him 
on his return, much new information which he imbod- 
ied in several works. It was he who explained to the 
Greeks, and caused them to adopt, the famous cycle 
of nineteen years, imagined by Meton, to reconcile the 
movements of the sun and the moon. The year of 
this cycle is still indicated, in our calendars, under the 
name of Golden Number. 

The sciences are all allied, and they mutually aid each 
other. Astronomy has lent its assistance to both Phi- 
losophy and Geography. Aristotle determined, by 
astronomical observations, the figure and magnitude of 
the earth. The proof of its sphericity he deduced 
from the appearance of the shadow which it projects, 
circularly, in eclipses, upon the disk of the moon; and 
from the inequality of meridian solar heights, in dif- 
ferent latitudes. 

Thus was the science of astronomy extended and 
improved, in the hands of these celebrated men. But 
among all the schools of antiquity, in which this sci- 
ence was taught, that of Alexandria shown preemi- 
nent. This school collected, with intelligence and care, 
a crowd of observations made by its pupils, with trig- 
onometrical instruments. At this school, too, were des- 
cribed, with care, the constellations; and it determined, 
in a precise manner, the position of the stars, and the 
course of the planets; and commenced an investiga- 
tion of the inequalities in the movements of the sun 
and moon. Hipparchus here determined the length of 
the tropical year with greater precision than had been 
previously done — differing no more than about four 
and a half minutes from its true value. 

Ptolemy, regarded as the first of astronomers, lived 



44 HISTORY OF ASTRONOMY. 

in the second century of our era. He has transmitted 
to us, in his Syntax, a work of great labour, the obser- 
vations and the principal discoveries of the ancients. 
He gave, in that work, the theory and the tables of the 
movements of the sun, the moon, the planets and the 
fixed stars. He had adopted the system which suppo- 
ses the earth placed in the centre of the world, and to 
which his name has been given. The errours which 
his system embraced did not prevent this great man's 
calculating the eclipses which were to take place in the 
ensuing six hundred years. 

The Syntax of Ptolemy was translated, about the 
year 826, of our era, by the Arabs, who gave it the 
name of Almagest. Four centuries afterwards this 
translation was rendered into latin, by order of Fred- 
erick II. Alphonso X, king of Castile, afterwards 
assembled the principal known astronomers, and caus- 
ed them to prepare new tables, which are hence called 
Alphonsine. . 

This protection strikingly impressed the enlightened 
men of Europe. Astronomy was cultivated because it 
conducted to preferment, reputation and favours. — 
Treatises upon the science were multiplied, and with 
them the instruments which facilitated observations. — 
But the most remarkable event of this epoch is the re- 
production of the ancient system of the world, discov- 
ered by Pythagoras. It was Copernicus, born at 
Thorn, in 1473, who resuscitated this system. He 
found that the system of Ptolemy, which supposed the 
earth fixed, and the sun, moon and planets turning, in 
concentrick circles, around this body, did not accord 
with observed phenomena ; and he saw that the diffi- 
culties which encumbered that system would disappear 
in admitting that the sun is a centre, around which the 
earth, with the other planets, makes its annual revolu- 
tion. This theory rests upon reasoning so incontesta- 
ble that none other is now taught, or deemed worthy of 



HISTORY OF ASTRONOMY. 45 

credence. Unfortunately Copernicus had not the sat- 
isfaction to witness the triumph of the doctrine which 
he had so well defended. Persecuted by zealots, and 
ridiculed by the learned, it was long after he had pre- 
pared the work in which he detailed his observations 
and discoveries, before he published it to the world. — 
When he finally did so, he lived to see a copy of the 
published work, but died a few days after. 

The only serious opposition which the theory of Co- 
pernicus experienced was from Tycho-Brahe, the cel- 
ebrated Danish astronomer, who combated the Coper- 
nican system of the universe, to make room for his 
own. His system differed little from that of Ptolemy; 
yet it bears the name of its Danish defender. He sup- 
posed that the earth was the centre of the world, and 
that the sun accomplished around it a revolution in 
twenty-four hours. The other planets, also, by this 
system, revolved round the earth in a peculiar and 
somewhat complicated manner. Some of his disci- 
ples, nevertheless, supposed the earth endowed with a 
diurnal motion upon its axis; and that the sun and all 
the planets made their revolutions round the earth in 
one year. We shall demonstrate the errour of this 
hypothesis in speaking of the system of Copernicus. 

One of the scholars of Tycho-Brahe, namely, Kep- 
ler, added much to the rapid progress of astronomy. 
Hipparchus, Ptolemy, and even Copernicus, owed a 
great part of their knowledge to the Egyptians, the 
Chaldeans and the Indians: they all followed a beaten 
track. But Kepler was indebted solely to his own 
genius for those discoveries which have rendered him 
so celebrated: antiquity had bequeathed him no traces 
which indicated the route he pursued. 

Galileo lived at the same epoch. While one of these 

great men traced the orbits of the planets and unveiled 

the laws of their movements, the other subjected to his 

researches the general laws of motion, which had been 

4* 



46 DEFINITIONS. 

two thousand years neglected, it was through the aid 
furnished by the labours of these two philosophers that 
Newton and Huygens were subsequently enabled to 
determine all the planetary movements. Galileo had 
demonstrated, in the most incontcstible manner, that 
the earth had a diurnal and an annual motion; but this 
was contrary to the received doctrines of the day. 
The cardinals summoned him before them to answer 
for so grave a crime, and without regard to his age, 
his virtues, or his extended knowledge, they condemned 
him to perpetual imprisonment. 

Since Newton, who greatly perfected it, astronomy 
has not ceased to be cultivated by men whose profound 
knowledge and valuable discoveries have greatly illus- 
trated the science; but we have neither time or space 
farther to devote to this division of our subject. 

DEFINITIONS. 

Astronomy treats of the movements, distances, mag- 
nitudes, physical constitution, eclipses and all other 
phenomena of the celestial bodies. 

Under the general name of stars are vulgarly com- 
prehended all those bodies, save the sun and moon, 
which deck the celestial spaces; but astronomy arran- 
ges them in several classes. 

It distinguishes as fixed stars those which, in the 
revolutionary movement of the celestial sphere, ap- 
pear always to preserve the same relative position; 
maintaining, among themselves, apparent invariable 
distances. To recognise these, and to designate them 
with greater facility, astronomers have divided them 
into groups, to which they have given the name of 
Const ell atiojis. Each of these has its particular de- 
nomination, derived from the name of a man, an ani- 
mal, &c; which is sometimes indicated by the form 
of the constellation itself, but is almost always capri- 



DEFINITIONS. 



47 



ciously chosen. The utility of these denominations 
has perpetuated them among us. In order to distin- 
guish from each other the individual stars of each con- 
stellation, they are classed according to their light, or 
apparent magnitude, giving to each a particular desig- 
nation. The French mark the principal star of each 
constellation with a capital A; and the remaining stars 
are marked according to the method employed by John 
Bayer, in the celestial Charts which he published; 
which consists in designating each of them, in the 
order of its magnitude, by the letters of the Greek 
alphabet, commencing with a for the first or principal 
star, /3 for the second, &c. If the number of letters 
in the Greek alphabet prove insufficient, the Roman 
alphabet is then substituted, after which, if there is 
still a deficiency, it is supplied by ordinary figures, 
1, 2, 3, &c. This method has been generally followed 
by all modern astronomers. 

Since, therefore, all maps and globes which exhibit 
the stars, will be found to have these Greek references 
attached to them, it is necessary that they should be 
within the reach of the student, and they are conse- 
quently subjoined. 

THE GREEK ALPHABET. 



LETTER. NAME. 

a Alpha, 


SOUND. 

a 


LETTER 

V 


. NAME. 

Nu, 


SOUND. 

n 


/3§ Beta, 


b 


I 


Xi, 


X 


y f Gamma, 


s 





Omicron 


o shor 


5 Delta, 


d 


<A Zj 


Pi, 


P 


s Epsilon, 


e short. 


2 P 


Rho, 


r 


?§ Zeta, 


z 


tf s 


Sigma, 


s 


-/) Eta, 


e long. 


c7 


Tau, 


I 


& 6 Theta, 


th 


U 


Upsilon, 


u 


t Iota, 


i 





Phi, 


ph 


x Kappa, 


k 


X 


Chi, 


ch 


X Lambda, 


1 


* 


Psi, 


ps 


fx Mu, 


m 


w 


Omega, 


o long. 



48 DEFINITIONS. 

Observations having detected the fact that certain 
heavenly bodies, aside from their apparent motion of 
diurnal revolution, were subject to another and actual 
motion, which caused the distances from them to sur- 
rounding stars to vary, thereby clearly showing that 
they changed places in the heavens, such were desig- 
nated errant stars, or planets. 

Primary planets may be defined opaque, celestial 
bodies, which revolve round the sun, as their centre, in 
orbits nearly circular, and which shine by the reflected 
light they receive from that body. 

Beside the primary planets there arc others, called 
secondary, or more commonly, satellites or moons, 
which revolve round some primary planet, as a centre, 
and are carried, with that planet, by attraction, in its 
revolution round the sun. 

The primary planets are farthermore divided into 
superiour and inferiour. The superiour planets are 
those which are more elongated from the sun than is 
the earth: they are, -Mars, Vesta, Juno, Ceres, Pallas, 
Jupiter, Saturn and Uranus: the inferiour are those 
whose distance from the sun is less than that of the 
earth: they are, Mercury and Venus. 

It was proposed, by Herschel, to refuse the name of 
planet to Vesta, Juno, Ceres, and Pallas, on account 
of their diminutive size; but the proposition has not 
been generally sanctioned. These planets are not 
smaller, in comparison with Mercury, than Mercury 
is in comparison with Jupiter; and as they differ not, 
in other respects, they should manifestly bear the name 
planet — which is now generally accorded to them, as 
well defining their character, and avoiding unneces- 
sary confusion. 

The following are the names of the several planets 
now known to belong to our solar system, with the 
sign or character usually employed to designate each. 



DEFINITIONS. 49 

These are sometimes divided into ancient and modern; 
and we will, therefore, so distinguish them. 

PLANETS KNOWN TO THE ANCIENTS. 

: Mercury, $ Mars, 

: Venus, 21 Jupiter. 

© Earth, \ Saturn. 

PLANETS DISCOVERED BY THE MODERNS. 

n§. Uranus, 2 Juno, 

9 Ceres, g Vesta 

$ Pallas, 

The orbit of a celestial body is the trajectory which 
it describes in its revolution round that body which 
serves it as a centre. The orbits of the planets are 
ellipses of very feeble excentricity; while those of com- 
ets on the contrary, have much greater excentricity, 
and consequently deviate much more from a circle. 

The ellipse is any section of a cone, made by a 
plane oblique to, and not encountering the base of that 
cone. It may readily be produced by fixing firmly at 
two points, a thread, not fully extended, and then pass- 
ing a pencil round these two points, all the while press- 
ing it against the thread, so as to hold that fully 
extended. When the line thus produced, (the pencil 
having been guided, throughout, by pressing against 
the thread,) shall have been joined at all points, the 
curve thus obtained is that known as an ellipse. 

Suppose, figure 17, A and B two fixed points, to 
which are attached the two ends of a thread, A C B, 
flexible, but not elastick, and longer than the interval 
A B. If this thread be extended by a fine point, C, 
its two parts will form, at pleasure, either the triangle 
A C B, in which A C and B C are equal; or the 
triangles A D B, A E B, &c, in which the sides A D 



50 



DEFINITIONS. 



and B D, A E and B E, &c, will, on the contrary, 
be more or less unequal in proportion as the point shall 
be nearer to L or to M. 

Fig. 17. 








In passing from the right to the left of the line A B, 
the point, in its progress, will give rise to a series of 
triangles, similar to those upon the opposite side. In 
each of these series of triangles the sum of the dis- 



DEFINITIONS. 51 

tanccs of the moving point C, from the two fixed points 
A and B, will be always the same, for this sum con- 
stitutes the entire length of the thread. 

Among all the positions through which the descri- 
bing point, in this process, passes, there are two which 
merit our particular attention. The}' are those in 
which the triangles formed by the base A B and the 
two portions of the extended thread become straight 
lines; that is, when the describing point is either at 
L or M, in prolongation of the line A B. 

We will first suppose the describing point at L. 
The thread is now extended, in a right line, from B to 
L; there it passes round this point and returns, in the 
same direction, from L to A, where the end was first 
fastened. In this position there are two portions of 
the thread, between L and A, applied to the same line; 
and the distance, therefore, from B to L is equal to 
the entire length of the thread, less the distance from 
A to L, through which, as we have shown, the thread 
is double. "When the describing point is at M the cir- 
cumstances are the same. From A to M the distance- 
will be, likewise, equal to the length of the thread, 
diminished by the distance from M to B. Now M B 
cannot differ, in length, from A L, because the two 
ends of the curve must exactly correspond. Then, if 
to the distance B L, which is less than the entire 
length of the thread, by the quantity A L, we add 
either A L, or its equal, B M, the distance thus ob- 
tained will be that of the entire length of the thread.. 
Thus A L, added to B L, gives us the distance M L^ 
or, in other words, the distance of the two extreme 
positions of the describing point, situated upon the 
right line A B, is equal to the total length of the 
thread. 

Geometricians call that curve which the point C 
thus traces an ellipse. Artists, gardeners, mechan- 



52 DEFINITIONS. 

icks, &c, call it an oval; and they arc in the daily 
habit of producing it, in the manner here described. 

This curve is elongated, in the direction of the right 
line which joins the points A B. 

The points A and B are called the foci of the ellipse. 

The line M L is the transverse, or major axis. 

The points M and L, where the transverse axis 
touches the curve, are the vertices. 

The point O, situated in the middle of A B, and 
consequently in the middle of M L, is the centre of the 
curve; but this term. ; centre, it must be borne in mind, 
has not the same meaning, here, as it has when ap- 
plied to a circle, for all the parts of an ellipse are not 
equally distant from it, as those of a circle are. 

The line extending across the ellipse, from C in the 
direction of O, and perpendicular to the line A B, is 
the conjugate or minor axis. 

The distance between the centre, O, and the focus 
A; or the distance, which is the same, from O to the 
focus B, is called the excentricity of the ellipse; and 
the less this excentricity is, the nearer the ellipse ap- 
proaches to a circle. 

Let the points A and L, remain unmoved, and con- 
ceive the other focus, B, and the other vertex, M, to be 
removed, in the direction of the axis A B, to greater 
and greater successive distances. In that case these 
several new positions of B and M will correspond to 
new ellipses, all of which will enclose the first. When, 
by an abstraction which we realize in calculation, the 
locus B, is thus removed to an infinite distance; or, in 
other words, when the ellipse has an infinite trans- 
verse axis, it then takes the name of parabola: a curve 
which is not closed, but has two infinite branches, as 
P P. At L, and for some distance upon either side of 
this point, the two curves, the ellipse and parabola, 
nearly coincide. They both gradually depart farther 



DEFINITIONS. 



53 



and farther from the line which joins the two foci; and 
in the case of the ellipse, the maximum of this dis- 
tance is attained at the extremity of the conjugate 
axis. Beyond this the ellipse again approaches the 
grand axis, which it finally encounters at M. But 
it is not so with the parabola; the branches of which 
are separated farther and farther from each other, so 
long as their extension is continued. The deviation 
of one of these curves from the other becomes first 
sensible at a greater and greater distance from the 
vertex L, according as the transverse axis is more 
and more prolonged. 

Any ellipse is said to be completely determined,, 
when the two foci and the transverse axis are given; 
since, from these data, such ellipse may be produced. 
In proof of this it is only necessary to recollect that 
the transverse axis is the total length of the thread 
used in describing the ellipse, and that the foci are the 
two points where the ends of this thread are to be con- 
fined. 

There is also a very simple rule by which to pro- 
Fig. 18. duce any ellipse of which the 
transverse and conjugate axes, 
only, are given. To illustrate 
this, suppose, Fig. 18, A B the 
given transverse axis, and C D 
the given conjugate axis. Then, 
to describe an ellipse whose 
axes shall be these given lines, 
/ first, from the centre, F, de- 
scribe the semicircle A E B. 
Then apply the length of the 
conjugate axis, C D, from A to 
the point where that distance 
will touch the semicircle A E 
B. In this figure it touches at E. Then, from this 
point, draw the line E B, and the length of this line is 




54 DEFINITIONS. 

die distance between the foci of the ellipse required: 
in other words, the distance beween the two points at 
which the ends of the thread, whose entire length is 
A B, are to be confined. 

The ecliptick is the orbit described, in appearance, 
by the sun round the earth, but in reality by the earth 
round the sun. 

Sensible horizon, a circle which separates the visible 
from the invisible hemisphere; or the circle which is 
the boundary of our sight. 

Rational horizon is a plane passing through the 
centre of the earth, and parallel to the sensible 
horizon. 

Azimuth, is an arch of the horizon comprised between 
the meridian and the vertical plane which passes 
through an object. 

Colures are ancient names by which we designate 
two great circles of the sphere, which pass, that of 
the equinoxes, through the equinoctial points and the 
pole of the equator; that of the solstices, through the 
solstitial points and the poles of the ecliptick and the 
equator. 

Terrestial longitude is the angle of the meridians, 
measured by the arch comprised between them, upon 
the equator. The longitude of a star is the arch of the 
ecliptick comprised between that star and the point °p. 

Terrestrial latitude is the distance of a place from 
the equator, computed upon its meridian; and the lati- 
tude of a star is the distance of this star from the eclip- 
tick, measured upon a great circle which passes through 
the star, and the pole of the ecliptick. 

Two planets are in conjunction when they have the 
same longitude; and they are in opposition when their 
longitudes differ 180°. 

Declination is the distance of a heavenly body from 
the equator, measured on a meridian; and it is either 
north or south. 



DEFINITIONS. 



55 



Meridian, a great circle of the earth, passing- through 
both poles; the plane of this circle extend 
sphere of the heavens, marks out the celestial meri- 
dian: the plane of the meridian is the plane of this 
circle; and its intersection with the sensible horizon 
constitutes a meridian line, or a north and south line 
from any given point to the horizon of that point. 

The zenith is the summit of the celestial vault, 
the point directly over our heads, one of the poles of 
the horizon. 

The nadir is the point opposite to that last described; 
the inferiour pole of the horizon. 

The Poles are the extremities of the axis of a circle. 

The Nodes are the points where the orbit of a pla- 
net cuts or crosses the ecliptick. The node from which 
a planet elevates itself towards the north, above the 
plane of the ecliptick, is the ascending node; that from 
which it descends towards the south is the descending 
node; and a line from one of these to the other is the 
line of the nodes. 

The Solstices are the two extreme points of the 
sun's apparent excursion to the north and south of the 
equator. 

The Tropicte are imaginary circles, parallel to the 
equator, which are touched by the sun at its greatest 
distance north and south, thus marking the limits of 
the torrid zone. 

The Sphere is the apparent concave surface which 
surrounds our globe, and in which we seem to view 
the celestial bodies. This appears to turn upon the 
two poles. 

Apogee is that point in the orbit of a planet where 
that body is at its greatest distance from the earth; 
and Perigee the opposite to this, or the point where it 
is nearest the earth. 

The Apsides are those points in the orbit of a pla- 
net where that body is either at its greatest or least 



56 DEFINITIONS. 

distance from ihe sun or the earth. The first of these 
points, that is, the one most distant, is called the Aphe- 
lion, and the other Perihelion. The line which joins 
these two points, passing through the centre of the sun, 
Arc. is the line of the Apsides. 

&y~ygy is the common denomination of the opposi- 
tion and the conjunction of the moon, relatively to the 
sun. 

The Equator is a great circle of which all the 
points are at an equal distance from the poles. 

Positions of the sphere are its situations with refe- 
rence to the horizon: they are right, parallel and 
oblique. The right position of the sphere is where the 
poles are in the horizon, and the equator is in the ze- 
nith. People on the equator, only, have this position. 
A parallel sphere, is where the equator coincides with 
the horizon, which is only at the two poles of the earth. 
In all other positions the sphere is oblique. 

The Parabola is a section of a cone cut parallel to 
ihe side of that cone, and encountering its base: it is, 
consequently, an open curve, the two ends of which, 
by prolongation, would never meet. See page 52. 

Parallax is the angle comprised between the direc- 
tions in which a heavenly body would be seen, if 
viewed at the same instant from the centre of the 
earth and from a point upon its surface. In other 
words, it is the apparent change of place, of a hea- 
venly body, caused by a real change of place of the 
observer, situated upon the earth. We will illustrate 
this by a diagram. Suppose the circle T O D B, fig. 
19. to represent the earth. M the moon, S the sun, 
and E X L E a portion of the ecliptick. C would 
be the centre of the earth: O B two observers placed 
at these two distant points upon the earth, for the pur- 
pose of observing the parallax of two heavenly bodies; 
say the sun and moon. The line H shows the hori- 
zon of the observer, O. and the line V his zenith; 



DEFINITIONS. 



57 



while the lines A and W, in like manner, denote those 
of the other observer, B. Then, the observer at O 



Fig. 19. 

True 
Place. 










* 
* 



/\ i A 




would see the sun, among the fixed stars, at L, while 
the observer at B would refer the same body to the 
point N of the heavens. These two observations, 
then, would give different results, as to the position of 
5* 



58 DEFINITIONS. 

the sun, at the same moment; and this difference would 
be the total effect of parallax, with reference to these 
two positions; or the value of the angle O S B, formed 
by the rays O S and B S, at the centre of the sun. 
This angle once ascertained, we can determine there- 
from the angle which either of these rays forms with 
the vertical line C S, and consequently we obtain the 
horizontal parallax of the sun. 

So of the moon. The same observer, placed at O, 
refers the moon to the point E of the ecliptick; while 
the one at B, sees the same body at the point E'. By 
this it is shown that the parallax is greater, in propor- 
tion as the planet or star is nearer to the earth; in 
other words, that the apparent place of any one of 
these bodies differs more from the true place, under 
these circumstances. But it must be borne in mind 
that any one of these bodies, if situated in the zenith, 
or directly over the head of the observer, (as if he were 
stationed at T,) has no parallax, but is seen in its true 
place, as if viewed from the centre of the earth, at C. 

The Zodiack is a zone of about eighteen degrees in 
width, divided in its breadth, by the ecliptick, into two 
equal portions. In its circumference it is divided into 
twelve parts, called signs; and each of these signs is 
divided into thirty degrees. The signs of the zodiack 
have each received a specifick name; and each is also 
represented, in celestial globes, maps, &c. by a cer- 
tain character, or symbol. These arc as follows: 



0. 


Sign. 


Latin name. 


English name. 


Degrees. 





T 


Aries, 


the Ram, 


0° 


1 


« 


Taurus, 


the Bull, 


30° 


2 


n 


Gemini, 


the Twins, 


60° 


8 


35 


Cancer, 


the Crab, 


90° 


4 


SI 


Leo, 


the Lion, 


120° 


5 


■m 


Virgo, 


the Virgin, 


150° 


6 




Libra, 


the Balance, 


180° 


7 


n 


Scorpio, 


the Scorpion, 


210° 



DEFINITIONS. 59 

No. Sign. Latin name. English name. Degrees. 

8 / Sagittarius the Archer, 240° 

9 K5 Capricornus, the Goat, 270° 

10 £? Aquarius, the Water Bearer, 300° 

11 X Pisces, the Fishes, 330° 

These signs are situated in the order in which they 
are here named, proceeding from west to east; and 
this is designated the order of the signs. 

The etymological explanation of these several 
names has given rise to numerous and protracted 
discussions, to which the researches of the Institute 
of Egypt, at Paris, have finally put an end, by show- 
ing that these names, now adopted by all people who 
devote any attention to astronomy, were drawn from 
comparisons made by the Egyptians between celestial 
and terrestrial phenomena, purely local, for the most 
part, and appertaining exclusively to a particular por- 
tion of their country. The following is an abridg- 
ment from the researches in question, which cannot 
fail both to interest and instruct. 

1st. Sign op the Goat, (Capricornus,) ]£?. 

This is the first month of summer, and extends 
from the 20th of June to the 20th of July, about. 

In Greek, Etkjm, g<rij<p/, (according to Alberti, Fa- 
hricii Menologium. ) 

Coptick, Epep, (Lexicon Mgyptiano-Latinum of 
Lacroze.) 

Arabick, Hebhebi, Hebheb. 

Latin. The definition of these several names may 
be thus rendered; Caper, dux gregis, qui ccepit, spe- 
cies apparens aqua, evigilatio, motio hue et illuc, 
aurora. 

The Arab word hebheb, or habeb, signifies cazpit, 



60 DEFINITIONS. 

evigilavit, experrectus fait e somno, flavit ventus, va- 
cillavit, hue et illuc motus fuiU insiliit infavellam. 

The following is an explanation of the Latin phra- 
ses which have served as a translation of the ideas 
expressed by the Coptick and Arabick words. 

Caper, the name Capricorn, Goat, one of the twelve 
signs of the Zodiack. 

Dux gregis, qui cozpit. Capricorn opens, or com- 
mences the year; he is the chief of the Celestial ani- 
mals, as, upon the earth, he is that of the flock of 
which he constitutes a part. 

Species apparens aqua, the commencement of the 
increase or flood of the Nile, which is not ordinarily 
appreciable until ten days after the solstice. 

Qui evigilavit, qui experrectus fuit e somno, desig- 
nates the longest day: the sun, or the animal which 
represents it, is awakened and roused at an hour which 
is consecrated to sleep in the other seasons of the year. 

Qui vacillavit, qui hue et illuc motus fuit, the appa- 
rent hesitancy of the sun, in its motion, when arrived 
at the solstice. 

Quijiavit ventus, a north wind which blows during 
fifteen days, at this epoch. The Egyptian almanacks 
note the period of its commencement. 

Aurora: This proves that the Egyptian year com- 
menced at this period. Finally, according to Herodi- 
tus, Epiphi, or Epephi was probably one of the twelve 
astronomical gods of the Egyptians, for he says, 
book II, chapter 38, that they esteem bulls as sacred 
to this deity. 

2d. Sign op the Water Bearer, (Aquarius,) £?* 

This is the second month of summer, and extends 
from the 20th of July to the 20th of August. 

Greek, Mstfopi, Mstftfopi, Msrfwpi, Mftfop*], (Menolog.) 
Coptick, Mesore. 



DEFINITIONS. 61 

Arabick, Mesour, misr. 

Latin, Vas aqua, paulatim lac suum reddens 

The Arabick word meser may be rendered prcebuit 
paulatim, emulsit quidquid esset in ubere. 

The addition of the i final, which personifies me- 
sour i, causes it to signify aquarium. 

Paulatim lac suum reddens, &c. coincides perfectly 
with the picture of the Water Bearer, in the Zodiacks 
of Essori and Denderah, where the vase, slightly in- 
clined, surfers the water which it contains to escape, 
or overflow. 

Emulsit quidquid in ubere. It is mostly during this 
month that the Nile pours out the bounty of her fer- 
tilizing waters. The Egyptians figuratively regard 
the waters of these inundations as milk, in sweetness 
and nutrition, so important are they to the fecundity 
of their lands. The inundation continues to increase 
during this month. 

3d. Sign of the Fishes, (Pisces,) X. 

This is the third month, from the 20th of August to 
the 20th of September. 

Greek, Tw0, Swud, SwSi, pSw. 

Coptick, Thoout. 

Arabick, Thohout. 

Latin, Ambulatio pisces, incessus, reciprocatus 
ultro, retroque in se rediens. 

The Arabick word tona may be rendered peragra- 
vit regionem, opplevit puteum. The hout, fish; hat, 
circumnatavit, 

Ambulatio, &c. show us the fishes going and re- 
turning to and fro, in the waters which overflow and 
cover the country. 

Opplevit puteum, designates the inundation spread 
over all the low grounds; for, at this time, it extends 
over all Egypt. Finally the festival of Isis is placed 



62 DEF^TTIONS. 

at the commencement of this month, because it is only 
then that the people celebrated the festival of the Nile, 
or the opening of the dikes. It is for this reason that 
it has been sometimes ca\\ed fotouh, apertura per terra 
superjiciem fluentis aqua, opening of the dikes. 

A passage of Sanchoniathon, preserved by Philo, 
says that messori gave birth to Thoth; and we see 
that it is messori, or the increase of the Nile, which 
produced touhout, the expansion of the waters upon 
the face of Egypt, in which the fishes were enabled 
to disport themselves over the fields. 

4th. Sign op the Ram, (Aries,) c p. 

Aries is the first month of Autumn; commencing on 
the 20th September, and terminating on the 20th of 
October. 

Greek, <£awp», tfoecpi, cacopi. 

Coptick, Paopi. 

Arabick, Fofo,foafi. 

Latin, Hcedus, velox, vox qua greges increpantur. 

The Arabick word is rendered by increpuit gre- 
gem dicensfafa. 

The Hebrew word fafa signifies obtenebrescere. 

Vox qua greges increpantur. As the waters of the 
flood withdrew from the land, the Ram once more con- 
ducted to the fields the flock of which he was the head, 
and which had been held in captivity by the waters, 
during the inundation. 

Ohtenebrescere. The increase of darkness, through 
the diminution of the length of days, from the autum- 
nal equinox. 

5th. Sign of the Bull, (Taurus,) tf. 

The Bull, second month of Autumn, from the 20th 
of October to the 20th of November. 



DEFINITIONS. 63 

Greek, ASwp, ahzpi, (cuwp, Eusebius.) 

Coptick, Athor. 

Arabick, Thaur, athour. 

Latin, Taurus, Tauri. 

The word athor may be rendered aravit, submovit 
terram, in reference to the season of ploughing, which, 
in Egypt, does not commence until seed time is past, 
in some other countries, in the month of November. 

6th. Sign of the Twins, (Gemini,) n« 

The Twins, third month of Autumn, from the 20th 
of November to the 20th of December. 

Greek, Xcax, %oiax, Kocu, Krpcog. 

Coptick, Choiak. 

Arabick, Chauk. 

Latin, Amor e flagrant ss, amatores. 

In the Egyptian zodiacks this sign is indicated by 
the figures of a young man and a young woman: 
during this month seed grain, which has been sown, 
swells and germinates: it was therefore an errour of 
the Greeks to name this sign, as they did, dioojxoi, the 
Twins, as the emblems employed by the Egyptians 
were intended to indicate the season of fecundity. 

7th. Sign of the Crab, (Cancer,) 55. 

Cancer is the first month of winter, from the 20th 
of December to the 20th of January. 

Greek, To&. 

Coptick, Tobi. 

The word teby may be rendered amovit, avertit; 
and the word teb, by reversus, conversus fuit, respuit. 

These roots well characterize the retrograde move- 
ment of the sun, at the solstice of winter. 



64 DEFINITIONS. 

8th. Sign of the Lion, (Leo,) St- 

The Lion, second month of winter, from the 20th 
of January to the 20th of February. 

Greek, Ms^ip, Ms^sjp, Mj^oj. 

Coptick, Chery or Mechery. 

The word cher may be rendered acquisivit, collegit; 
mecher by pars segetis; or mecher by protulit frondes, 
ramos; amcher, by plantas suas extulit terra injlatus, 
turgides fecit. 

All this is typical of the fact that, it is in the month 
of February that, in Egypt, the earth presents its 
most rich and imposing features, some part of the har- 
vest being then commenced. It is by a figure of the 
king of beasts that the Egyptians here typically per- 
sonified the force and ripening magnificence of nature, 
at this important period. 

9th. Sign of the Virgin, (Virgo,) frj£. 

This is the third month of winter, from the 20th of 
February to the 20th of March. 

Greek, 3>a|xsvw0. 

Coptick, Famenoth. 

Arabick, Faminoth. 

Latin, Mulier feeonda et pulchra, qucevendit spicam, 
frumentum, et quod portatur inter duos digitos. 

This word is composed of famij, a vender of the 
ears of grain, of every kind, of which the ears or the 
stalks can be carried between two fingers, and ofEnoth, 
a handsome, fruitful woman. In the Egyptian zodi- 
acks Famenoth, or the fruitful woman, holds an ear of 
grain in her hand. The Greeks were induced to the 
errour of naming this sign tfapSsvo^, the Virgin, by 
the fact that the Egyptian word conveys the idea of 
endowed with beauty; but that word also as distinctly 
conveys the idea of fecundity — the typical signification 



DEFINITIONS- 65 

of which, no less than the sense itself, the Greeks in 
this instance neglected or overlooked. 

10th. Sign of the Balance, (Libra,) ^=. 

The Balance, first month of spring, from the 20th 
of March to the 20th of April. 

Greek, $ap|xov$h 

Coptick, and Arabick, Paramour; which may be 
rendered by mensura, regula confecta temporis. 

This word has reference to the vernal equinox, and 
the consequent equality in the length of the days and 
nights. 

11th. Sign of the Scorpion, (Scorpio,) tt[. 

The Scorpion, second month of spring, from the 
20th of April to the 20th of May. 

Greek, Ila^wv. 

Coptick, Pachous. 

Arabick, Bachony. 

This word is composed of bach, rendered prostravit, 
humi stravit, which, in all the eastern languages, sig- 
nifies putruit; Icesit, pravis fuit, or putrido, malum, 
morbus; and of honniy, rendered venerium, aculeus 
Scorpionis et terror. 

The original here, is significantly characteristick of 
the second month of spring, when the increasing heat 
calls forth venomous beasts and reptiles from their 
retreats, to roam over the land; and also develops 
maladies and pestilence. The root hama signifies, also, 
ferbuit dies; the days are become burning, or torrid. 

12th. Sign of the Archer, (^Sagittarius,^ f. 

The Archer, third month of spring, from the 20th 
of May to the 20th of June. 
6 



66 DEFINITIONS. 

Greek, IIai)v«, tfawvi. 

Coptick, Paons. 

Arabick, Faync or fenni. 

Latin, Extremiias seculi temporis, horoz. Faijnan, 
fenan, may be rendered nomen equi, Onager varii cur- 
sus. 

The root fann signifies propellit, impulit; faijni sig- 
nifies propulsator, impulsator. 

Extremitas, last month of the Egyptian year. 

Nomen equi. Onager, name of a quadruped. Pro- 
pulsator indicates its action. In the Egyptian Zodiacks 
the figure of this animal has the body of a quadruped, 
and a head with two faces, the one of a lion, and the 
other of a man. The man is armed, and in the act of 
letting fly an arrow. This compound being seems dri- 
ving before him the animals in his front, while, with 
the other face, he frowns back those which appear to 
follow him. All indicates that this figure is typical of 
the close of the year, and that it is thus represented as 
having achieved its course. 



APPARENT MOTIONS OF THE CELESTIAL BODIES. 67 



CHAPTER THIRD. 

ASPECT OF THE HEAVENS APPARENT AIOTIONS OF 

THE CELESTIAL BODIES. 

Upon casting our eyes upward, in the open air, we 
behold spread out above our heads a vast concave 
hemisphere, of which we seem to occupy the centre, 
and which appears, in descending upon every side, to 
unite itself to the horizon. During the day this im- 
mense vault is enlightened by a brilliant disk, which, 
issuing from the regions of the east, moves majestically 
upward until noon, and then descending, upon the op- 
posite side, is lost to our view, at evening, below the 
horizon in the west. The feeble light which now re- 
mains is soon extinguished, and then appear, upon 
every side, in the immensity of space, a multitude of 
brilliant points, of various magnitudes, and of which 
the number increases as the obscurity of night becomes 
more profound. The motion of these bodies adds still 
farther to the beauty of the scene. While some of 
them, moving in the same direction as the sun, sink 
behind the western horizon and disappear; others, 
rising in the east, pass over the celestial vault, and dis- 
appear, in their turn, where the sun was so recently 
lost to our view. All, however, do not thus disappear 
from us, for there are those which, in their entire 
revolution, sink not below our horizon, but of which 
we may follow the motion during the whole night: and 
there is one which appears at all times immoveable. 
On the other hand, while some describe immense cir- 
cles, in the heavens, others pass over but a small arch 
of the horizon; while still another class do no more 
than just appear, and then immediately sink again be- 
yond our sight Such are the phenomena of the rising 
and setting of the stars. It is to this general move- 



G8 ASPECT OF THE HEAVENS. 

ment of the starry sphere, a single entire revolution of 
which is accomplished in a day and a night, or in 
twenty-four hours, that, the name of diurnal motion has 
been given. 

In this revolution of the sphere, the heavenly bodies, 
subject to the motion we have described appear, at first 
sight, to preserve among themselves the same relative 
distances from each other. But more precise observa- 
tions do not fail to show that, if this is true of the 
greater part of these bodies, namely, that the relative 
positions of them, and their respective distances asun- 
der are subject to no appreciable change, yet that 
some few of them are endowed with an actual motion, 
peculiar to themselves, which transports them succes- 
sively from one constellation to another. It is this- 
movement, or displacement among the stars, that we 
call the proper motion of the planets. 

The sun, like the planets, is endowed with a pro- 
per motion; since we see it rise and set successively, 
in different points of the horizon. At the end of June 
it rises far north, remains a long time above the hori- 
zon and at noon approaches nearer to our zenith; 
while at the end of December it emerges from the ho- 
rizon at a point much farther south, rises not, at mid- 
day, so near to our zenith, and describes, in the hea- 
vens, but a small circle above the horizon. It is to this 
movement that we are indebted for the variety of the 
seasons, and the inequality in the length of our days 
and nights. 

The motions of the moon, and the aspects which it 
presents, at the different periods of its course, are still 
more remarkable. This body first shows itself or be- 
comes visible, as new moon, in the western part of the- 
heavens, at a short distance from the sun, under the 
form of a crescent, which increases in size in propor- 
tion as the moon elongates itself from the sun, by pas- 
sing eastward in its orbit, until at length this body ri- 



APPARENT MOTIONS OP THE CELESTIAL BODIES. 69 

ses m the east at the same moment the sun is setting 
in the west. This is the period of full moon; and the 
face or disk of that body is then exactly circular. 
Continuing its movement eastward, it rises later and 
later each night, until it approximates the sun in the 
east, as it did, at new moon, in the west. It then 
shows itself, in the morning, a little before, or west of 
that luminary, as, in the first part of its course, it did 
in the west, a short distance behind, or east of him. 
These diverse phases all succeed each other in the 
course of a lunar month, to be repeated, in the succee- 
ding one, in the same order. 

Sometimes, indeed, we observe in the heavens lumi- 
nous bodies totally different from any of those which 
have hitherto occupied our attention; and which, from 
the various changes to which they are subject, have 
ever been objects of curiosity and astonishment to the 
people. Small, and but slightly luminous, when first 
visible, they nevertheless soon assume considerable 
dimensions, and often exhibit a luminous tail like ap- 
pearance, of which the extent and brightness are very 
variable. These bodies are Comets. They are en- 
dowed with a proper motion, the direction of which, 
unlike that of the planets, is susceptible of material 
change. As these bodies approach the sun they in- 
crease in light, and sometimes the train becomes 
greatly lengthened, more luminous, and more dis- 
tinctly defined. As they recede from the sun both 
their light and magnitude decrease more or less ra- 
pidly, until they finally disappear, entirely, from our 
view. 

In reflecting upon this revolutionary motion of the 
sphere, two questions very naturally present them- 
selves to the mind, namely, does each star always ac- 
complish its revolution in the same time? and, is its 
motion uniform; or, in other words, does it pass over 
equal spaces in equal times? 
6* 



70 ASPECT OP THE HEAVEXS. 

For the solution of the first of these queries it is suf- 
ficient to direct towards any star, whatever, a tele- 
scope, immoveably fixed, in some convenient position. 
By computing, accurately, the time which elapses un- 
til the reappearance of the same star in the telescope, 
we easily assure ourselves that the duration of the re- 
volution is absolutely the same, whenever tried, and 
whatever star may have been employed for the obser- 
vation. The space of time which elapses between two 
consecutive returns of a star to the same meridian, 
constitutes the siderial day. 

The second question is resolved by means of an ap- 
paratus designated a parallactick instrument. It is 
composed of a graduated circle, fixed to a central axis, 
perpendicular to its plane; the prolongation of this axis 
confounds itself with the diameter of another moveable 
circle, which is constantly maintained perpendicular 
to the first; this second circle, furnished with a tele- 
scope susceptible of assuming any inclination, in refer- 
ence to the central axis, in turning upon this axis, 
gives motion to a pointer which indicates, upon the 
first circle, the horizontal arches which it has passed 
over. Let us suppose this telescope directed towards 
some star which is constantly visible, (for it is neces- 
sary, for this, that the star be not lost sight of, during 
its entire revolution,) the axis of the instrument being 
parallel to that of the heavens, and a motion be com- 
municated to the moveable plane, corresponding to the 
motion of the star. This done, if we note, with rigid 
accuracy, the intervals of time which elapse while the 
moveable plane passes over equal arches of the fixed 
plane, we shall find that these intervals are equal to 
each other. In order, then, to appreciate the quantity 
of a stars motion, or displacement, we may take, in- 
differently, either the measure of the arch it has des- 
cribed, or the time which it employed to describe it, 
after having once esinblished, between these two data a 



APPARENT MOTIONS OF THE CELESTIAL BODIES. 71 

known and definite relation. Thus, the sphere accom- 
plishing its revolution in twenty-four hours, and all the 
diurnal circles being divided into three hundred and 
sixty degrees, the stars are found to pass over fifteen 
of these degrees during each hour. But it must be 
recollected that these different circles are not all equal, 
that their divisions, therefore, do not coincide, and 
that, to compare the results, it is necessary to deter- 
mine their relative value. 

It is a very common errour, extensively accredited, 
that any of the stars may be seen, in day-light, from 
the bottom of a well. They cannot generally be 
seen, during the day, except with the assistance of 
telescopes, or by ascending high in the air, upon some 
elevated mountain, or in a balloon. This is not true 
of the planets, whose light is much more considerable; 
and Venus, for example, may often be seen, with the 
naked eye, and in open day, while the sun is still 
many degrees above the western horizon, although 
she is always in the same quarter of the heavens as 
that luminary. The reason of our inability to see 
these bodies, in day-light, with the naked eye is, the 
rays of the sun, reflected in every direction by the 
atmosphere, form a kind of luminous curtain, through 
which the light of the stars, being comparatively very 
feeble, is unable to penetrate. It is sufficient, indeed, 
that, of two lights, in presence of each other, one of 
them be sixty times more intense than the other, in 
order to render the lesser light imperceptible to the 
eye. This fact may be verified by a very simple expe- 
riment: place between two lighted candles, a body 
which will project two shadows; then remove one of 
the candles to such a distance that the light which it 
sheds upon the intermediate body may be only one six- 
tieth of what it was at first: a result easily produced, 
when it is recollected that the intensity of light is in 
the inverse ratio of the square of the distances. The 



72 ASPECT OF THE HEAVENS. 

shadow produced by the light thus removed will not be 
visible, while at rest; but if motion be given to it, the 
shadow will then become perceptible. This is the prin- 
cipal reason why, with optical instruments, the stars 
are visible in full day; for these magnifying instru- 
ments, augmenting prodigiously their distances, accele- 
rate, by the like quantity, their motions. 

Aside from the proper motion of planets and comets, 
by which these bodies are distinguishable from the 
fixed stars, there is another difference which can hardly 
fail to be noticed by all. This is a scintillation, or 
rather twinkling which is peculiar to the fixed stars, 
and which consists in a change of intensity, accompa- 
nied by a change of colour, of these stars. The bet- 
ter to appreciate this it is necessary to refer to a re- 
markable discovery, recently made, in the properties 
of light. If we cause to fall upon the same point two 
luminous rays, having the same origin, they will not 
always add to the quantity of light; but it will some- 
times happen, if the rays be caused to pass over 
different distances, or through mediums of different 
densities, that, in certain conditions, these two rays, 
instead of aiding, destroy each other; so that we may- 
produce, however singular the result may appear, ob- 
scurity by adding light to light. This is the pheno- 
menon of luminous interferences; and it is by it that 
the twinkling of the fixed stars is explained. The 
different parts of the atmosphere being subject to a 
continual variation of density, realizes the conditions 
of the phenomenon of interferences, and intercepts, 
thus, some of the rays which compose the white light 
of the stars, leaving others to reach our eye, which 
can only produce a feeble and diverse coloured image. 
The planets do not exhibit this appearance, because 
they have a visible surface, of appreciable extent. 

The aspect of the heavens varies with the position 
of the observer. Suppose he occupied exactly one of 



APPARENT MOTIONS OP THE CELESTIAL BODIES. 73 

the poles of the earth; for example, the north pole. 
In that position his zenith will be the celestial north 
pole, and his rational horizon will coincide with the 
equator. All the stars having a northern declination, 
that is to say, all those which are comprised between 
the north pole and the equator, will appear to describe 
circles parallel to the horizon; those occupying the 
equator will constantly move in the horizon, while all 
those having a southern declination will remain con- 
stantly invisible. The parallelism of all these motions, 
with reference to the horizon, has caused this position, 
as we have already remarked, to be designated the 
parallel sphere. 

Let the observer now be transported to the equator: 
his rational horizon will then pass through both the 
poles, and, in this position, he will have all the stars 
in view during the time they employ in describing one 
half their diurnal circles, that is, while the sun is be- 
low the horizon, and the planes of these circles will 
be perpendicular to the horizon. This is the right 
position of the sphere. 

If the observer now proceed from the equator towards 
one of the poles, the north pole, for example, this pole 
will appear to elevate itself gradually above the hori- 
zon, and the south pole will be depressed below the 
horizon, in the same proportion. Suppose, for exam- 
ple, that he has proceeded thirty degrees towards the 
arctick pole, his zenith will be C F, fig. 20. The 
great circle, H O R, will be his horizon; the plane of 
the equator, E O Z, will be elongated from the zenith, 
F, thirty degrees, and consequently will be distant 
from the horizon sixty degrees. The pole, P, will be 
elevated thirty degrees, measured by the angle H C P, 
and the pole P' will be abased, by an equal quantity, 
below this plane. It follows, from this demonstration, 
that the distance from the zenith to the equinoctial, or 
the latitude, is always equal to the height of the pole 



74 



ASPECT OF THE HEAVENS. 



above the horizon. In this position the circles descri- 
bed by the stars are inclined to the horizon; from 
which circumstance this position is called the oblique 
sphere. 

Fig. 20. 
p 




In following the course of the stars of the sphere we 
have seen them all successively elevate themselves 
above the horizon and subsequently sink below it. At 
what point, then, do the stars cease to ascend? and 
how shall we determine, accurately, that point? There 
are several methods which conduct to this result: the 
following, founded upon the correspondent heights of 
the sun, is, perhaps the most simple. 

Upon a surface exactly horizontal (which is to be 
determined by means of the ordinary spirit level) erect 
a stile that shall be strictly perpendicular, and around 
the foot of this, as a common centre, describe several 
circles, of different diameters. Then, on each of these, 
mark carefully, and accurately, before and after noon, 
the exact point at which the extremity of the shadow 
that is cast by the stile, touches upon that circle. 
By dividing the arch comprised between the two points 



APPARENT MOTIONS OF THE CELESTIAL BODIES. 75 

which the shadow has traced upon each circumference, 
we obtain a line which, passing through the foot of the 
stile, determines the plane in which the sun was situa- 
ted, when it had attained the highest point of its course. 
This instrument is called a gnomon, and the plane 
which it serves to determine is the meridian. It passes 
through the zenith of the place, and through the poles; 
and a right line traced upon the earth, in this plane, 
is a meridian line, and cuts the horizon at the same 
point as the celestial meridian. 

Another method, also very simple, is that of the mea- 
sure of time; but it is necessary for this purpose, to 
make use of a transit instrument, or meridian tele- 
scope, which, as it is frequently employed by astrono- 
mers, we will describe. 

This instrument, like the astronomical telescope, is 
composed of a cylindrical tube to which are fitted an 
object glass and eye piece. At the focus of the object 
glass is placed an opaque screen or partition, having 
a small perforation in the centre, through which the 
central or axis rays may pass to the eye, while all 
others are intercepted, in order to render the vision 
more perfect and defined. At the same point are dis- 
posed, upon moveable, metallick mounting, some ex- 
ceedingly fine threads of metal, glass or other sub- 
stance, which are so adjusted as to divide the field of 
view into four equal parts. In the micrometer these 
threads are usually five in number, placed vertically 
and parallel, while a sixth is adjusted horizontally, at 
right angles to these. The transit instrument, fixed 
firmly upon trunnions, is so constructed as to be im- 
moveable, except only in a vertical plane. 

To determine the meridian, place the instrument in 
a vertical plane, direct the telescope to some star which 
is constantly visible, through its whole revolution, ob- 
serve the respective instants of its greatest and least 
elevation, and count, by a very exact and well regula- 



76 ASPECT OF THE HEAVENS. 

ted clock, the time which elapses between the two pas- 
sages of the star. Usually, however carefully we may 
have chosen the plane, there will be found a great dif- 
ference in the time consumed by the star, in passing 
over the respective distances between these two points, 
and upon opposite sides of them; the one requiring 
more time than is demanded by half a revolution, that 
is, more than twelve siderial hours, and the other less. 
It will suffice, then, to know this difference, and to ad- 
just the instrument in that plane which will divide, ex- 
actly into two halves, the diurnal circle of the star; 
which may easily be done by successive trials. To 
illustrate this, suppose cfd e, fig. 21, the diurnal cir- 
Fig. 21. 



/ 



/ 



bd, 

cle of the star; and that, at the first trial, the transit 
instrument be adjusted in the vertical plane a b. The 
star will then consume less time in passing through 
af 5, than it will in passing through b e a. But when 
the instrument is subsequently adjusted in the vertical 
plane c d, which, passing through the centre of the 
diurnal circle, divides it exactly into two halves, the 
time through one will then be exactly the same as 
that through the other. 

There are, also, different methods by which to fix 
and determine the position of the stars. Of these, two 
are more particularly employed. 



APPARENT MOTIONS OF THE CELESTIAL BODIES. 77 



The first consists in measuring the angles formed 
by vertical planes passing through each star, with a 
meridian to which is referred the distances of these 
stars. 

It is commenced by determining the height of the ob- 
served star in the vertical plane in which it is placed, by 
the aid of the quadrant of a mural circle. This is a 
sector, furnished with a moveable telescope, at the focus 
of which is fixed a micrometer, composed of two move- 
able threads, only; one vertical, the other horizontal. 
The radius of the circle should be disposed vertically, 
in the plane of the meridian, and should correspond to 
zero of the divisions traced upon the dial plate descri- 
bed by the radius. The vertical thread of the micro- 
meter serves to direct the optical axis in the plane of 
the radius, an indispensable condition, in order that the 
arches measured by the limb may be equal to those 
described by the optical axis. From the moment that 
the axis enters into the field of the telescope, by means 
of convenient machinery, it is made to follow the ho- 
rizontal thread, and when its centre touches the verti- 
cal thread it is exactly in the plane of the meridian. 
Then read off, upon the graduation, the arch which 
the angle formed by the vertical radius and the visual 
radius measures, and this angle is the distance to the 
zenith; the complement of the meridian altitude. 

It is next requisite to know the angle comprised be- 
tween the vertical in which the observed star is situa- 
ted, and the meridian: this angle is called the azimuth 
of this star; and it is eastern or western. It may be 
arrived at by noting exactly the time of its passage of 
the meridian, and in the vertical where it is observed: 
then the time which elapsed between these two passa- 
ges, will give its value. This method, which is quite 
simple, has been very frequently employed. 

The zenith distance and the azimuth of a star, ele- 
ments necessary to fix its position, may also be obtained 
7 



78 ASPECT OF THE HEAVENS. 

by the aid of an instrument composed of two graduated 
circles, of which the horizontal one will indicate the 
meridian line, and the other, furnished with a tele- 
scope and micrometer, is perpendicular to the first, 
and moves round a vertical which passes through its 
centre. When a star is to be observed, bring it to the 
centre of the cross threads, having previously care- 
fully adjusted in its vertical plane the circle last men- 
tioned. It will indicate, then, the height of the star 
above the horizon, and its distance from the zenith, 
which is its complement, while the horizontal or azi- 
muth circle marks the azimuth, at the moment of the 
observation. 

The zenith distances and the azimuths form, then, 
as we see, a system of angles, by the aid of which it is 
very easy to obtain the position of the stars, in a rigo- 
rous manner. But this method presents an inconve- 
nience, in practice, which has caused it to be almost 
entirely rejected: which is, that the zenith and the 
azimuths varying as often as the observer changes his 
horizon and his meridian, there remains no fixed point 
to which we are able to refer all the observations, and 
the different positions offer no common point of com- 
parison. It is for this reason that the* following, known 
as the method of determining the right ascensions and 
tlie declinations of the stars, is now generally preferred 
and practised. 

By this process it suffices to know the hour circle 
of the star, and its position upon this circle. The po- 
sition of the star, upon the hour circle, is determined by 
the instrument which serves us to measure meridian 
altitudes; and the star's declination is its distance from 
the equator, reckoned upon its hour circle: hence we 
sometimes call these, circles of declination. 

Declination is reckoned from zero to 90°, or a right 
angle; and we call it northern, or southern, according 
as the star is north or south of the equator. 



APPARENT MOTIONS OP THE CELESTIAL BODIES. 79 

As to the position of the hour circle, it is determi- 
ned by the angle which it makes with a designated 
hour circle. If the angle formed by the intersection 
of these planes is measured by an arch of the equator, 
this arch is what we designate the right ascension. It 
is determined by observing the time which elapses be- 
tween the moment when the star passes the meridian, 
and the passage of the hour circle which has been cho- 
sen as the point of departure. Astronomers designate 
by the sign SP the point from which to reckon the right 
ascension of the stars; and this point is that where the 
sun crosses the equator, when it ascends from the 
southern tropick towards the north. 

Right ascension, then, is the angle which the hour 
circle of a star forms with the meridian, at the instant 
when the fixed point of Aries, % the point where the 
sun appears to us to enter upon spring, is in the plane 
of the meridian. Right ascension is always computed 
from west to east, upon the equator, and from zero to 
360°, or the entire circumference. This system of 
lines, bv which we determine the position of the stars, 
offers, as will be readily perceived, much analogy 
with the preceding, yet it differs essentially in this, 
that the positions of the stars being taken with refer- 
ence to circles of the celestial sphere which are invari- 
ably fixed, (for, indeed, they are no other than the ce- 
lestial equator and a fixed meridian,) all the observers 
situated upon the surface of the earth are enabled to 
refer their observations to these, and to compare with 
each other the results they have obtained. The decli- 
nation and right ascension once known, we readily find 
all the relations of situation and of distance upon the 
celestial sphere. 

What we have shown upon this subject will be suf- 
ficient to make known how we are enabled to obtain a 
catalogue of the stars, by means of a meridian tele- 
scope or any other convenient, instrument. We first 



80 OP THE FIXED STARS. 

determine the instant in which any star, whatever, 
which we know, passes the meridian; and we note ex- 
actly the hour, minute and second of its passage. We 
do the same with each of the other stars, in succession, 
as they arrive in the plane of the meridian. In this 
manner we determine their right ascensions, and at 
the same time the exact height or declination of each 
one of them. These data once acquired, it is easy to 
indicate the position which they should preserve, rela- 
tively to each other; and thus we should have a celes- 
tial chart, upon which would be traced the different 
groupes of stars which form the constellations. 

The first celestial charts are very ancient. Hippar- 
chus is the first who is known to have constructed 
them; and as the relative distances of the stars have 
undergone no sensible change since the earliest obser- 
vations, these charts may always be employed in a 
study of the heavens. 

The point at which we commence to compute right 
ascension, serves us also for that of siderial time; that 
is to say, we count Oh 0' 0" siderial time; at the mo- 
ment of its passing the meridian. 

From this is seen that nothing is more easy than to 
know the hour, in siderial time, the height of the pole 
in the place where one observes being previously 
known. It suffices to observe the zenith distance of a 
known star, and to calculate its hour angle, computing, 
for example, from the superiour meridian, and in the 
direction of the diurnal motion, from 0° to 360°, ad- 
ding this angle to the right ascension of the star, and 
rejecting the entire circumferences, if any. The re- 
mainder, converted into time, will express the distance 
of the meridian from the point of the heavens that has 
been assumed as the point of departure, that is to say, 
the siderial hour. 



OF THE FIXED STARS. 81 

CHAPTER FOURTH. 

OF THE FIXED STARS. 

We have already stated that under this general de- 
nomination are comprised all the celestial bodies which 
appear to maintain, invariably, their relative positions. 
We say appear, because modern observations, particu- 
larly those of Herschel, father and son, Bessel, 
Struve, &c, show changes in their mutual relations 
from which it is certain that some, at least, of the fixed 
stars, have proper motions, though apparently so slow 
as to be almost imperceptible. Among those fixed 
stars which appear but single to the naked eye, there 
are many that, in the telescope, under a high magni- 
fying power, are resolved into two or more stars, lying 
but little out of the right line of vision, although per- 
haps often at very different distances from us. These 
are called double, triple, or multiple stars, according 
as they may be resolved into two, three, or more; and 
their number is very great. A catalogue of these, 
but embracing only those of a certain description, and 
extending no farther than to 15° of southern declina- 
tion, was published by Prof. Struve, of the Dorpat 
Observatory, Russia, in 1827, that contains the places 
of no less than 3112 of these stars. 

Motion is detected in some of these, which, although 
called fixed, are found to have regular revolutions 
around each other, in well defined orbits. The num- 
ber of such now certainly known, is about forty; al- 
though observations have by no means been so nume- 
rous, upon this subject, as to enable us to fix their 
number at no more than this. Among these may be 
noticed £ of Cancer, and g of Ursa Major, which 
have nearly completed an entire revolution since they 
were first observed; and r\ of the Crown has probably 
made more than one such revolution; while Castor, y 
7* 



82 OP THE FIXED STARS. 

of Virgo, <j of the Crown, 70 of Ophiuchus, and 61 
Cygnus are unquestionably of this class, and have 
changed their positions remarkably. The last of these, 
from observations made as late as 1838, is believed to 
accomplish its revolution in something more than 540 
years. 

The number of the fixed stars at first appears im- 
mense, because they are confusedly scattered over 
the celestial sphere, and cannot all be embraced, at 
once, in the field of vision. But it is easy to con- 
vince ourselves that the number of those visible to the 
naked eye is very limited, and does not exceed, at 
most, a few thousands. This may be shown by taking 
any one portion of the heavens separately, and count- 
ing all the visible stars it contains: they will be found 
but few; yet, by the assistance of telescopes their 
number may be multiplied beyond all known limits, 
save that of the size and power of the instrument em- 
ployed. 

Their distribution in the heavens, in groups or clus- 
ters, probably suggested the first idea of dividing them 
into constellations. We have already seen that these 
are systems of stars distinguished from each other by- 
letters and figures. Hipparchus has transmitted to us 
a general table of the constellations which were known 
in his time. They were 43 in number: 12 in the 
zodiack, 21 northern and 15 southern. Since his 
time their number has been considerably augmented. 

The following table contains the names, both classi- 
cal and English, of the several constellations, and the 
number of stars that each contains, according to the 
respective catalogues of Ptolemy, Tycho-Brahe, Hev- 
eleus and Bode. The Nebuke in each, as far as de- 
termined, are also noted; though improved telescopes 
may very probably much augment the number of 
these, as they doubtless will, and in some instances 
already have, that of the stars themselves. 



OF THE FIXED STARS. 



83 







Number of Stars in 


CLASSICAL NAMES 


ENGLISH NAMES 






: J . 










M © 


raw a 








^ Sr> 


a 3 


© u 


- ra EjQ 


W 


OF THE 


OF THE 


¥3 




© s 


^£ o 




CONSTELLATIONS. 


CONSTELLATIONS. 


z*V 




^u 


U 





NORTHEN, ANCIENT CONSTELLATIONS. 



Urea Minor, 

Ursa Major, 

Draco, 

Cepheus, 

Bootes, 

Corona Borealis, 

Hercules, 

Lyra, 

Cygnue, 

Cassiopeia, 

Perseus et Caput 

Medusa, 
Auriga, 
Ophiuchue, 
Serpens, 
Aquila, 
Delphinus, 
Equuleus, 
Pegasus, 
Andromeda, 
Triangula, 
Coma Berenices, 



The Little Bear, 
The Great Bear, 
The Dragon, 
Cepheus, 
Bootes, 

Northern Crown, 
Hercules, 
The Harp, 
The Swan, 
Cassiopeia, 
Perseus and 

Medusa's Head, 
The Charioteer", 
Serpent Bearer, 
The Serpent, 
The Eagle, 
The Dolphin, 
The Little Horse, 
TheWinged Horse 
Andromedia, 
The Triangles, 
Berinece's Hair, 









26 


35 


29 


73 


156 


81 


32 


40 


126 


13 


4 


51 


98 


23 


18 


52 


125 

26 


29 


28 




89 


10 


11 


17 


39 


19 


18 


47 


112 


13 


46 


33 


55 


29 


29 


46 


95 


14 


9 


40 


79 


£9 


15 


40 


124 


18 


13 


22 


84 


15 


19 


42 


85 


10 


10 


14 


26 
15 


20 


19 


38 


14 


23 


23 


47 


62 


4 


4 


12 


19 


14 


14 


21 


65 



NORTHERN, MODERN CONSTELLATIONS. 



Leo Minor, 
Canes Venatici, 
Sextans, 
Cerberus, 



The Little Lion, 
The Grayhounds, 
The Sextant, 
Three headed Dog* 



TaurusPoniatowskiiBull of Ponatowski, 
Vulpecula et Anser[The Fox and Goose 



35 


14 


21 


49 




o 


23 

V 


39 

40 

13 






29 


44 



4 

2 

10 

2 

4 

2 

18 

12 

6 
10 
13 



9 



*This Constellation was formed, by Heveleus, from part of 
Hercules; and it is still, by many, treated as part of that constel- 
lation. It is sometimes figured, in maps and upon globes, as a 
three headed dog, and at others as three serpents, in the left hand 
of Hercules. 



84 



OP THE FIXED STARS. 







Number of Stars in 


CLASSICAL NAMES 


ENGLISH NAMES 




-Z 












30 £ 


d e 


(E QD 


m ° 










c 3 






■ 


OF THE 


OF THE 


E i L 


£5 o 


« 5b 


*io H 


"Z3 






-^ 


C 


> ~~ 


S"§ 


X> 


CONSTELLATIONS. 


CONSTELLATIONS. 


— gd 


13 r^ 


— * 


^ "e 


?^ 






o*Q 




~U 


<J 





Lacerta, 
Mueca Borealis, 
Tarandus, 

Custos Messium, 

Camelopardalis, 

Lynx, 



| The Lizard, 
j The Northern Fly, 
I The Rain Deer, 
The Guardian of 

the Harvests, 
The Cameleopard, 
Lynx, 



25 



25 



10 21 3 



CONSTELLATIONS OF THE ZODIACK. 



°P_ Aries, 

Taurus, 

Gemini, 
95 Cancer, 
6~lLeo, 
-TTJ2. Virgo, 
— Libra, 
1T[ Scorpio, 
I Sagittarius, 
Y3 Capricornus, 
££ Aquarius, 
y^ Pisces, 



The Ram, 
The Bull, 
The Twins, 
The Crab, 
The Lion, 
The Virgin, 
The Scales, 
The Scorpion, 
The Archer, 
The Goat, 
The Water Bearer, 
The Fishes, 



21 
43 

15 

33 

10 

8 

14 

2S 
41 
36 



SOUTHERN, ANCIENT CONSTELLATIONS. 



Cetus, 

Eridanus, 

Orion, 

Lepus, 

Canis Minor, 

Canis Major, 

Argo Navis, 

Hydra, 

Crater, 

Corvus, 

Centaurus, 

Lupus, 

Ara, 

Corona Australis, 

Piscis Australis, 



The Whale, 

The River Po, 

Orion, 

The Hare, 

The Little Dog, 

The Great Dog, 

The Ship Argo, 

The Water Serpent 

The Cup, 

The Crow, 

Centaur, 

The Wolf, 

The Altar, 

j Southern Crown, 

(Southern Fish, 



22 


21 


45 


127 


34 


10 


27 


97 


38 


42 


62 


107 
20 


2 


5 


13 


21 


29 


13 


21 


39 


45 


3 


4 


136 


27 


19 


31 


100 


7 


8 


10 


21 


7 


4 


4 


23 


37 






70 


19 






32 


7 






5 


13 






12 


18 






27 



OF THE FIXED STARS. 



85 







Number 01 jSiars iii 


CLASSICAL NAMES 


ENGLISH NAMES 




-n 














_a cj 


to c 






OF TILE 


OK THE 


_ OD 
c 






~c G 


jj 






-'- 






o "3 


J3 


CONSTELLATIONS. 


CONSTELLATIONS. 


S cd 


■83 


„~ «g 


"*"' c 


S 






— 


>.vJ 


T 


H 





SOUTHERN, MODERN CONSTELLALIOXS. 



Fornax Chemica 
Reticulus Rhom- 

boidalie, 
Apparatus Sculpto- 

ris, 
Dorado vel Xiphias. 
Horologium, 
Norma Euclidis, 
Circinus, 
Triangulum Aus- 

trale, 
Columba Noachi, 
Equuleus Pictorius, 
Monoceros, 
Pyxis Nautica, 
Autlia Pneumatics, 
L'oiseau Solitaire, 
Crux, 

Musca Australie, 
Chamelion, 
Piscis Volaus, 
Telescopium, 
Apus, vel Avis In 

dica, 
Moris Menese, 
Scutum Sobieski, 
Indus, 
Pavo, 
Octans, 
Microscopium, 
Grus, 
Touchana, 
Hydrus, 
Cela Sculptoria, 
Phoenix, 
Sceptrum, 



Cbymists' Furnace 

The Rhomboidal 
Net, 

The Sculptor's 
Tools, 

The Sword Fish, 

The Clock, 

Euclid's Square, 

The Compasses, 

Southern Trian- 
gle, 

Noah's Dove, 

Painter's Easel, 

Unicorn, 

Mariner's Compass. 

Air Pump, 

Bird of the Desert, 

The Cross, 

Southern Fly, 

The Chameleon, 

Flying Fish, 
jThe Telescope, 
IThe Bird of Para- 
I diss, 

Table Mountain, 

Sobieski's Shield, 
iThe Indian, 
jThe Peacock, 
(The Octant, 
|The Microscope, 
jThe Crane, 
'American Goose, 
! Water Snake, 
'Engraver's Tools, 
i Arabian Bird, 
IThe Sceptre, 



21 



12 



10 



3; 



13 



11 

SO 9 





38 


19 


59 
10 
23 

18 




25 


7 


19 
12 




17 


16 


10 

18 

7 

9 



30 
1 



11 






!ii.L~ m»- z_r^: mi.;- ii-"..:'--r ~: uf lm rvuLl" l.- 
te: mm: -i'i :>Mim -.- M'- _ lm M"-m 1:1- m.:_. ii- 
~j±-. '•■ : ' m- ■■:. ~ \:,- :n- " j m : ~ tm~mm; r:eLr::i M" 
:m~— m mm: .' m>- ::•" imimmmmm _-.; _ vMMi mh 
:':'•:_ Mi- mm mm - - r:i'. ~ :..- : m; 1 ::-_■■ \~ 
mc «d aaa. in snceessioii- 
; - - : m -- — imim " -:,t lui.-r:-:- :■:' m- mi.- 

\ v.n-:i= _::- vy-.: \ --::-• m- vmmm -.mm> 

: 

":_ : mmm : v 1 :V: : - _tm. 1- mm : : : 

_ .' 
■•: mj- m mmm. -mm:m'- ,1 !■ : M '~ r 
m 
m MeM mmm^ TV.m lm:m- u- 

'& t ; 
. liiM- -mmlmimmm ^mtvmtbI 

:e: _.- :■■.• m m. - rzLi'V-U m ■-._ m- M.r^ 
- tmm- -•"'■ — 1 r-rh: _zi±- :r~ m --.ir' ?i. 
-_--jje: _r..i; ?-^rL_ r mmm. v- :"::" 

is. m.imm 1? rr^L" -*i.\"±~— -- tie 1 

-■:' r -.'-- mm :•-■:.".. .r - m- :-:' :^s- 
: -mm Mil.' : . m. L " 'J t'.lHn vvmm. 







OP THE FIXED STARS- 87 

covered thirty-one nebulae, all distinctly visible, in a 
perfectly serene night. Their situations, their volume 
and their light offered a diversity hitherto unknown. 
In another strata, which is, perhaps, a different branch 
of the first, 1 have often seen double and triple nebulae, 
variously arranged; some appearing surrounded by a 
multitude of small bodies, like attendants; in others 
the nebulous light was narrow, but much extended; 
again this was in the shape of a fan, resembling an 
electrick brush, issuing from a luminous point; others 
were of the cometick shape, with a seeming nucleus 
in the centre; or, like cloudy stars, surrounded with a 
nebulous atmosphere; a different sort, again, contain 
a nebulosity of the milky kind, like that wonderful, 
inexplicable phenomenon about 6 of Orion; while 
others shine with a fainter, mottled kind of light, which 
denotes their being resolveable into stars. 

It is probable that the great stratum, called the 
Milky Way, is that in which the sun is placed, though 
perhaps not in the very centre of its thickness. YVe 
gather this from the appearance of this galaxy, which 
seems to encompass the whole heavens, as it certainly 
must do, if the sun is within the same. For, suppose a 
number of stars arranged between two parallel planes, 
indefinitely extended every way, but at a given con- 
siderable distance from each other; and calling this a 
siderial stratum, an eye placed somewhere within it 
will see all the stars in the direction of the planes of 
the stratum projected into a great circle, which will 
appear lucid on account of the accumulation of the 
stars, while the rest of the heavens, at the sides, will 
only seem to be scattered over with constellations, 
more or less crowded, according to the distance of the 
planes, or the number of stars contained in the thick- 
ness or sides of the stratum. 

We are now able to appreciate the place which our 
little planet occupies in this vast universe. Take a 



88 OF THE FIXED STARS. 

single star of that immense system, and compare it to 
the innumerable quantity of the stars; and, in order to 
form a more correct judgement, examine it, at first, 
with the naked eye. The stars of the first magnitude, 
being probably nearest to us, furnish us the first degree 
of our scale: consequently, if we take the distance of 
Sirius or of Arcturus, for example, for unity, we may 
suppose that those stars of the second magnitude are 
at double the distance of these, while those of the third 
magnitude are at treble the distance; and so of the 
rest. If we admit that a star of the seventh magni- 
tude is about seven times as distant from us as one of 
the first, an observer placed in the centre of a sphere 
surrounded by stars could not discern the most distant 
parts of the stellary system, with the naked eye, be- 
cause, according to our estimate, such view could not 
extend beyond seven times the distance of Sirius, 
while the utmost limit of this system may be adjusted 
at, perhaps, very many times such distance.* The 
visible universe of such an observer would only in- 
clude the constellations, with all the stars that accom- 
pany them; unless, indeed, the night be one of unusual 
serenity, without clouds, haze, or other obstructions 
to vision, in which case some of the principle nebulous 
stars are sometimes discernible. But furnish this in- 
dividual with a telescope of only moderate power and 
he readily suspects that the Milky Way is but a vast 
accumulation of stars, a suspicion which is soon 
converted into certainty, by a farther augmentation of 
the power of his instrument. The numerous nebulae, 



*In this extract it has not been deemed expedient to change the 
language in which Herschel has clothed his fanciful theory res- 
pecting the fixed stars; and it is therefore necessary to advise the 
pupil that this theory, namely, that those fixed stars which appear 
the largest are nearest to us, cannot be true unless all those bodies 
are of the same magnitude, and emit the same quantity of light — 
of which there does not exist the least proof: nor has the supposi- 
tion even probability in its support. 



OF THE FIXED STARS. 



89 



also, scattered through the heavens, and, for the most 
part, visible only through the aid of telescopes, are 
likewise composed of clusters of stars, whose indivi- 
dual light, by reason of their proximity, is thus blended 
in one common mass." 

Herschel observed the Milky Way with much atten- 
tion, and he found that in its lightest portions the stars 
were immensely numerous. He states, (what seems 
impossible to know,) that in one quarter of an hour he 
once saw there one hundred and sixteen thousand stars 
pass across the field of view of his telescope, which 
had an aperture of only 15'; and that, at another time, 
in a period of only forty-one minutes, he thus counted 
258,000! With him, as with all subsequent astrono- 
mers, each successive improvement of the telescope, 
in size and power, has added to the number of stars 
actually observed; so that their number, and the extent 
they occupy, seems bounded only by the limits of the 
Universe. 

Our sun is probably no other than one of the fixed 
stars; since, if it were removed from us to the distance 
which it is known the fixed stars must be, it would 
present to us the same appearance as those bodies do. 
Hence we may conclude that these stars, which shine 
by their own light, since their distance is so enormous 
that it is impossible for them to receive their light from 
our sun, as the planets of our Solar System do, are 
comparable to our sun, both in brilliancy and volume; 
that their distances from each other are equal to their 
distances from us: and finally, reasoning from what 
we know of the mechanism of the heavens, that, like 
our sun, these bodies dispense light and heat to plane- 
tary systems which gravitate around them, but which, 
from their immeasurable distance, are rendered invisi- 
ble to us. 

Herschel thought that our sun N has a direct progres- 
sive movement, in space, towards the constellation 



90 OP THE FIXED STARS. 

Hercules, carrying, of ccirse, with it, our entire plane- 
tary system. He remarked that the apparent motions 
of forty-four stars, out of fifty-six which he had care- 
fully examined, were very nearly in the direction 
which would produce a real movement of this kind, in 
the solar system, and that the brilliant stars Sirius and 
Arcturus, have, as they should, if this theory be true, 
the greatest apparent motion. The star Castor, when 
viewed through a telescope, is shown to be two stars, 
of about equal magnitude; and although they may have 
an apparent motion, it has not been possible to detect 
a variation of a single second in the distance between 
them; and yet, this would be very easily done, if their 
apparent movement were owing to a real, progressive 
motion of our sun, through the regions of space, as 
Herschel supposed. 

In examining, with care, the catalogues of the stars 
which have been left us by the ancient astronomers, as 
those of Ptolemy, Tycho-Brahe, Heveleus, &c. we are 
struck with several very singular facts; namely, some 
of the stars have changed, more or less, their quantity 
of light; others have appeared, as new stars, where 
none had been known before, while others, still, have 
disappeared, entirely; some to return subsequently to 
view, with all their former brilliancy, and others that, 
when once lost sight of, have since been nowhere seen. 
These astonishing and inexplicable phenomena have 
not been confined to any particular period, but have 
been manifested at various, and distant times, accord- 
ing to the testimony of different witnesses, who have 
recorded the facts they have observed. Halley, the 
English astronomer, gave a very interesting account 
of these phenomena, which was published in the 
Transactions of the Ro}-al Society of London, in the 
year 1715. The following is abridged from that pub- 
lication. "The new star in the chair of Cassiopeia 
was not seen bv Cornelius Gemma, on the 8th of No- 



OF THE FIXED STARS. 9"i 

vember, 157*2, although he says he that night exami- 
ned that portion of the heavens, with care, during a 
very serene sky; but on the following night, Novem- 
ber the 9th, it appeared with a splendour which sur- 
passed all the fixed stars, and fell but little short of 
that of Venus. This was not seen by Tycho-Brahe 
until the eleventh day of the same month; but from 
that time he assures us that it gradually decreased and 
died away until, at the end of sixteen months, namely, 
in March, 1574, it was no longer visible. Its position 
in the heavens, according to the observations Of Ty- 
cho-Brahe, was, in right ascension,, 9° 17', and north- 
ern declination 53° 45'. 

On the tenth of October, 1604, the scholars of Kep- 
ler discovered another new star, in the right leg of 
Serpentarius. It was not visible the previous night, 
but shone out, almost at once, with a lustre surpassing 
that of Jupiter. This, like the former one, faded gra- 
dually away, and finally disappeared entirely, in Janu- 
ary, 1606. These two nev/stars seem to have differed, 
materially, from any others. Neither of them had 
either tail or envelope, as comets usually have; nor 
had they any parallax; from which it is known that 
they were at a greater distance than any of the planets. 
But between these two periods, namely, in 1596, David 
Fabricius discovered another of these singular variable 
stars. It was situated in the neck of the Whale, and 
at the time of its greatest brilliancy equalled that of a 
star of the third magnitude. This body has since 
been found to exhibit periodical changes in the inten- 
sity of its light. But although variable in its appear- 
ance, it never becomes wholly extinguished, but con- 
tinues always visible, with the aid of the telescope. 
No star was known, in the heavens, resembling this in 
variableness, until the discovery of such an one in the 
neck of the Swan. Its right ascension was 1° 40'; 
southern declination 15° 57'. A new variable star 



92 OF THE FIXED STARS. 

was discovered in the neck of the Swan, in the year 
1600, by W. Janson. This did not exceed a star of 
the third magnitude; and after continuing for some 
years distinctly visible, it became so much diminished 
that it was thought, by many, to have entirely disap- 
peared; but during the years 1657, 1658, and 1659, 
it gradually rose again to a star of the third magni- 
tude. The period of its variation of magnitude is 
fixed, by modern astronomers, at about eighteen years. 
The right ascension of this star was 9 s 18° 38'; dec- 
lination north, 55° 29' minutes. In July, 1670, a fifth 
new star was first seen and observed by Heveleus. It 
then appeared like a star of the third magnitude, but 
by the beginning of October it was hardly to be seen, 
with the naked eye. In April following it was again 
as bright as before, or rather greater than a star of 
the third magnitude, and yet it wholly disappeared 
about the middle of August, of the same year. The 
next year, in March, 1672, it was again visible, but it 
did not then exceed a star of the sixth magnitude; and 
since that period it has not been seen. The position 
of this star was, in right ascension, 9 s 3° 17'; and 
northern declination 47° 28'. The sixth is that which 
was discovered by G. Kirch, in the year 1686. Its 
period of change was determined to be 404i days; 
and although it rarely exceeds in size, a star of the 
fifth magnitude, yet it is very regular in its returns, 
as was carefully observed, in the year 1714. On the 
15th of June, 1715, after long and careful watching, 
with a good telescope, it was perceived, and in appear- 
ance it resembled one of the very least telescopick 
stars. During the remainder of that month, and the 
July following, it increased so that in August it was 
visible to the naked eye, and thus remained until the 
end of September. After that it again diminished, by 
degrees, and on the 8th of December, at night, it was 
scarcely discernable, by the aid of a telescope. It 



OF THE FIXED STAItS. 93 

was seen, in all, this year, about six months, which is 
but little less than half its entire period. The time of 
its greatest brightness is about the 10th of September; 
and its position is, in right ascension, 9 s 6° 30': dec- 
lination north, 52° 40'.'" 

The star Algol, also, is a very remarkable body, 
belonging to the class of periodical stars. It changes, 
continually, from the first to the fourth magnitude; and 
the time employed, from its greatest to its least magni- 
tude, and including the interval of time in which it 
experiences no change, is a little less than three days. 
From its full splendour, as a star of the second mag- 
nitude, it is reduced to one of the fourth, in a little 
more than three and a half hours; it then increases, 
for a like period, when it again arrives at its former 
magnitude. These peculiarities were discovered by 
John Goodricke, who first began to observe them about 
the end of the year 1782, and they have been fully 
confirmed by subsequent astronomers. Those stars 
which, in the last century, were suspected to be varia- 
ble, are divided into two classes: in the first class are 
placed all those which are really changeable; and in 
the second, such as were only presumed to be so. 
The first class embraces twelve in number, from the 
first to the fourth magnitude; comprising that which 
appeared in Cassiopeia, in 1572, and the one of 1604, 
in Serpentarius. The second class numbers thirty 
stars, of from the first to the seventh magnitude. 

Ingenuity has been exhausted in endeavours to ex- 
plain these surprising changes. Newton supposed 
that the temporary augmentation of the light of these 
stars arose from an augmentation of combustible ma- 
terials, by the fall of some comet "upon the body of 
the star. This system of Newton, which assumed 
that comets are destined to become fuel, wherewith to 
feed the combustion of our and other suns, as we add 
wood or coal to a fire, to perpetuate it, is certainly 
8* 



94 OF THE FIXED STARS. 

very little in harmony with the means which nature 
elsewhere employs, or with the now more generally 
received probability of the electrical nature of the 
combustion of the celestial bodies. Maupertuis sup- 
posed that these stars are endowed with a rotary mo- 
tion, upon their axis, so rapid that the centrifugal force 
has given to them the figure of an exceedingly flat- 
tened spheroid; or rather, reduced them to a circular 
plane, with broad side surfaces, and having little thick- 
ness. Possessing this shape, a star might sometimes, 
by the position of its axis, present to our view the 
broad face of its disk, and at other periods, by a 
change in the position of its axis, would be reduced 
to a minute speck, as it were, by a presentation of its 
thin edge, only, to the eye of the observer. Others 
have thought that these changes were produced by 
dark spots upon the surfaces of the stars; and others, 
still, have suggested that these immensely distant 
bodies revolve in orbits so vast that, like comets, they 
are only visible to us while traversing those regions of 
their path which lie nearest to us. The prevailing 
opinion of the best astronomers, in regard to these 
periodick stars, now is, that only part of their surfaces 
are enlightened, and that, consequently, each one, in 
revolving upon its axis, alternately presents to us sides 
more and less illuminated; thus causing the apparent 
size of the body itself to vary. 

From the facts already adduced, in relation to these 
distant bodies, a reflection very naturally arises re- 
specting our own sun. That body, as we have already 
said, is a star; and it differs from others in apparent 
size, by being placed much nearer to us in space. Has 
this body ever undergone any of these, or analogous 
variations? And if it has experienced any of these 
great vicissitudes, what incalculable consequences may 
have resulted to our earth therefrom? These conside- 
rations should, perhaps, command the attention of 



OP THE FIXED STARS. 95 

such geologists as seek for the causes of those tremen- 
dous catastrophes of which our globe furnishes, in al- 
most every part, so many traces. 

It remains for us, in closing this chapter, to form, 
if possible, some idea of the distance which separates 
us from the fixed stars. Before entering upon this 
problem, however, some previous explanations are 
indispensable. 

The angle subtended by any object varies in the in- 
verse ratio of the distance of the object from the eye 
of the observer. This is one of the most elementary 
propositition in geometry; and is' daily illustrated, in 
the observation of objects upon the earth, around us. 

On the other hand, trigonometry makes known the 
relations which exist between the dimensions of an 
object, its distance, and the angle which it subtends: 
thus, an object which subtends an angle of 1° is at a 
distance equal to 57 t 3 q- 8 q- times its size; if the angle 
is of 1', the body is at the distance of 3438 times its 
size; and it is at the distance of 206,000 times its size, 
if the angle subtended is one of 1". 

This being understood, it is easy to see that, the 
diameter of the earth being known, if we can know the 
angle which it subtends; in other words, its apparent 
size, when viewed from the stars, this fact would dis- 
close to us the distance of those bodies. It is this an- 
gle which we designate parallax. For finding it we 
employ a method analogous to that for measuring the 
distances between terrestrial objects. This consists in 
assuming a base line, of a known extent, and then 
measuring the angle formed, at each extremity of this 
line, by the visual rays from the object in question. 
These angles ascertained we substract their sum from 
180°, and the remainder will give the angle sought, 
according to that well known and fruitful proposition 
in geometry, namely, that the three angles of a trian- 
gle are always equal to two right angles, or 180°. 



96 OF THE FIXED STARS. 

Proceeding in this way, and when we assume, as 
a base line, the radius, or even the diameter of the 
earth, the parallax thus obtained is not appreciable, in 
the case of the fixed stars; by which we know that the 
diameter of the earth, compared to the distance which 
separates us from the fixed stars, constitutes but an 
imperceptible quantity. 

But, because neither the radius of the earth, which 
is about four thousand miles, nor its diameter, which 
is about eight thousand, is a distance sufficient to fur- 
nish us any parallax upon the fixed stars, a more ex- 
tended base line has been resorted to, by astronomers, 
for that purpose. The mean distance of the earth 
from the sun being ninety-five millions of miles, it fol- 
lows that the diameter of the earth's orbit must be twice 
this sum, or one hundred and ninety millions of miles. 
The particular points in space, then, which the earth, 
in its annual journey round the sun, occupies upon any 
two given days which have an interval of half a year 
between them, must be the entire diameter of this or- 
bit, or one hundred and ninety millions of miles, in a 
right line one from the other. Of this immense dis- 
tance astronomers, by allowing intervals of six months 
between their observations of the stars, have contrived 
to avail themselves, as a base line. This is known as 
the great, or annual parallax: and the highest hopes 
were entertained that the plan would determine the dis- 
tance of the fixed stars. Several astronomers observed, 
with the greatest care, and with suitable instruments, 
at the vernal and autumnal equinoxes, the passage of 
the star y, of the Dragon, little doubting that the dia- 
meter of the earth's orbit w T ould give, by this mode of 
observation, an appreciable angle. But their hopes 
were disappointed: no angle was perceived. And yet, 
if an angle of only one second of a degree, ( 1",) had 
been found, this would show us that the star in ques- 
tion is one hundred thousand times one hundred and 



OF THE FIXED STARS. 97 

ninety millions of miles from the earth; and yet we 
should he cnahled to measure its volume! We know, 
then, that this star is farther from the earth than this 
immense distance, but how much farther we have not 
the means to determine. What subject of contempla- 
tion so suitable as this to aid us in forming a concep- 
tion of the immensity of the celestial spaces, especially 
if we suppose, as we justly may, that the thousands of 
stars which decorate the heavens, have spaces between 
themselves, some of which, at least, arc equal to their 
distances from us? Not only may this justly be sup- 
posed true, but it is even indisputable, notwithstanding 
that the stars, to an unreflecting observer, appear to 
be all situated upon a common surface. The reason 
of this is that every celestial object is referred, by us, 
to the inner surface of an immense hollow sphere, of 
indefinite radius, having our eye for its centre. This 
sphere, and consequently our horizon, we always carry 
with us, in all our change of place, upon the earth. 
Whatever our course may be, stars that, in front of us, 
were below our horizon, at starting, constantly rise to 
view, and elevate themselves more and more, as we 
advance, while behind us, such stars as were in sight 
at first as constantly sink below the horizon, and dis- 
appear; and all this while the imaginary sphere, of 
which we have spoken, is found to have moved with 
us, and still, as at first, to have our eye for its centre, 
and consequently, to be divided through that centre by 
the plane of our horizon. 

Observations upon the fixed stars, of a later date 
than those referred to above, and made, too, with in- 
struments still farther improved, have been more suc- 
cessful. Professor F. YV. Bessel, of the Konigsberg 
Observatory, in October, 1838, communicated to the 
publick that he had been able to obtain an annual 
parallax upon the double star 61 of the Swan, amount- 
ing to 0' .3136. This places that star at the distance of 



98 DISTANCES, ETC. OF THE PLANETS. 

657700 times ninety-five millions of miles from the 
earth; and light, which moves with a known velocity, 
employs ten years and three tenths to traverse this dis- 
tance. 

In addition to the stars already noticed, as having 
vanished from sight, some of the European Astrono- 
mers, in 1839, published to the world the then recent 
disappearance of no less than tune other small stars, 
whose places, in the heavens, had thus been left vacant. 
These changes are wholly inexplicable to us, in the 
present state of our knowledge; and they only bear 
witness to us of the extraordinary activity of nature, 
in these elongated portions of the universe. 



CHAPTER FIFTH. 

DISTANCES, ETC. OF THE PLANETS. 

However great the magnifying power of the tele- 
scopes we employ may be, the apparent diameter of 
the fixed stars is never augmented. They always ap- 
pear as an indivisible point. The planets, on the con- 
trary, present a disk, of which the diameter increases 
with the magnifying power of the telescope we employ. 
These facts are alone sufficient to convince us that the 
planets are much nearer to us than the fixed stars; 
while the micrometer proves to us that each of their 
distances varies, one time with another, by disclosing 
to us variations in their apparent dimensions. 

The moon, which by the same means is shown to 
be still nearer the earth, was early submitted to the 
appreciations of geometry. In 1751 Lacaille and La- 
lande, two French astronomers, proceeded, the one to 
Berlin, and the other to the Cape of Good Hope, for 
the purpose of determining the parallax of this body, 
and thereby fixing its distance from us. The pupil is 



DISTANCES, ETC. OF THE PLANETS. 99 

already informed that parallax is the angle formed by 
two visual rays departing from a planet or star, and 
terminating at the two extremities of the terrestrial 
radius. They found this angle, in the case of the 
moon, to be 1°, which gives, for the mean distance of 
the moon from the earth, about sixty times the earth's 
radius, or 240,000 miles. The diameter of the moon 
is a little more than one quarter that of our earth, and 
its volume is about one fiftieth that of the same planet- 
It is possible that an errour may exist, in the esti- 
mate of the moon's distance, by this method, amount- 
ing to half a second in each of the angles measured at 
Berlin and at the Cape, and consequently of one entire 
second, in the result, amounting to the three thousand 
six hundredth part of the entire distance, of 240,000 
miles. The possibility of this amount of errour, in all 
observations of this nature, is unavoidable, since angles 
cannot be thus measured with greater certainty. 

The parallax of the sun is 8' ,6 T T ¥ nearly, and its 
mean distance from the earth, ninety-five millions of 
miles, in round numbers. Its diameter is, to that of 
the earth, as 1 to 111, and its volume is in the propor- 
tion of 1 to 1,300,000. 

The parallax of the sun is known to one tenth of a 
second, very nearly; an approximation to the truth 
much greater than can be obtained by the measure of 
angles, as above described. This valuation, then, has 
been obtained by a different method; which we now 
proceed to explain. 

This knowledge is furnished by the passages of the 
planet Venus across the disk of the sun. Let S, Fig. 
22, be the sun, A B the radius of the earth, and v v' the 
planet Venus moving in its orbit round the sun. Now 
suppose two observers placed, one at A and the other 
at B, to observe and note, exactly, the different phases 
of the conjunction; the difference of their results 
would show the time which Venus consumed in pass- 



100 DISTANCES, ETC. OF THE PLANETS. 

ing over that portion of its orbit between v and v', and 
from this, though by a method too complicated to be pre- 
■p icr 22 sented here, is deduced 

__^J" the parallax of the sun. 

^"X This operation, which we 

/ have attempted to present 

/ 5 here, in its utmost simpli- 

* city, is, in practice, en- 

cumbered with difficulties 
arising from the constant 
motion of the earth, and 
various other particulars, 
all of which it is neces- 
sary rigidly to estimate 
and consider, in order to 
\b obtain a true and accu- 
rate result. 
The distances and the volumes of the other planets 
have been determined by analogous means; and we 
shall give the results of these, severally, while treat- 
ing of each planet separately, in the subsequen: 
oi this work. However, we will here make known 
the singular numerical relations which exist among 
the distances of the planets, with regard to each other. 
If we take the following numbers, namely. 0, 3, 0, 
12. 24, 4S. 96, 192, and to each of them we add the 
number 4, so as to obtain the following series, na. 
4, 7, 10, 16, 28, 52, 100, 196, these last quantities are 
found to express the order of the distances of the pla- 
nets from the sun, as follows: 

0, 3, 6, 1-2, 24. 
4, 7, 10, 16, 2g 

c - 

This singular proportion of the planetary distances. 
from the sun, is known as Bode's law, it having been 
discovered bv the late Professor Bode oi Berlin, 



48, 


96, 


192. 




100, 


196. 


o 


-- 


m 



DISTANCES, ETC. OF THE PLANETS. 101 

From the extent of space between the orbits of Mars 
and Jupiter, by which this law was very greatly vio- 
lated, that astronomer suggested that a planet was 
there wanting, and which might hereafter be found. 
Within the present century the four telescopick planets, 
namely, Vesta, Juno, Ceres and Pallas have been 
discovered, all revolving between Mars and Jupiter, 
and in orbits situated so nearly the same as to answer, 
very well, the requirements of this law. These facts 
early led to a strong presumption that the law in ques- 
tion was not the result of accidental circumstance, 
but one which entered essentially into the structure of 
the Universe: and such presumption seems now re- 
duced to entire certainty by the still more recent dis- 
covery that secondary planets, no less than primary, 
are adjusted at distances which are in conformity with 
this law. "This law/* 7 says Delambre, "which is 
not, indeed, very rigorously exact, is purely empirical, 
and no one is able even to suspect what its foundation 
may be." Investigations subsequent to those of De- 
lambre, however, have probably given us farther in- 
sight to this interesting subject. The late L. W. 
Caryl believed that, in 1837, he demonstrated the law 
in question to be one of those upon which rests the 
stability of the Solar System. If his demonstrations 
were not erroneous, then many of the planetary ine- 
qualities which now exist are due to the slight devia- 
tions, in the positions of the several planetary orbits, 
from a rigid observance of the law in question; and, 
farther, had the observance of this law been but 
slightly less than we find it to be, in those positions, 
the Solar System could not have maintained its place 
and adjustment, but must have gone to pieces, by the 
action of its several parts upon each other.* 

* Mr. Caryl died while still pursuing this interesting investiga- 
tion; and it is not known that his papers, in relation to it, were 
left in a condition to be available to others. He was a resident of 
Buffalo. 9 



10*3 DISTANCES. ETC. OF THE PLANETS. 

THE SUN, $. 

We have seen that the sun is an immense globe. 
1,300,000 times greater than the earth which we in- 
habit, and that its mean distance from the earth is 
95,000,000 of miles. We shall learn, in a future chap- 
ter, that attraction furnishes us the means of determin- 
ing its density, and its weight. We have already stated 
that, in the opinion of Herschel, this immense body is 
probably carried, with all the planets which revolve 
around it, through the regions of space, towards the 
constellation Hercules. Be this as it may, it certainly 
has a rotary motion upon its axis, by which it completes 
a revolution in twenty-five days. This is proved by ob- 
servations made upon the spots which its surface pre- 
sents, and of which we shall speak more fully when 
treating of the physical constitution of the sun. The pe- 
culiar motions of these spots, and the different aspects 
which they assume, according as they present them- 
selves obliquely, or directly in front, leave us no room to 
doubt that they pertain inherently to the surface of the 
sun; while they prove to us, incontestable", that the 
sun itself is a spherical body. We speak not, here, 
of the movement which the sun appears to execute, in 
the plane of the ecliptick: for this, as the pupil will 
learn, in a subsequent part of this work, is the result 
of the translation of the earth, through the different 
parts of its orbit. 

PHYSICAL CONSTITUTION OF THE SUN. 

The sun, as has been stated, presems spots upon its 
surface. Some of these are dark, and others luminous, 
but all differing, materially, in appearance, from the 
general complexion of the sun. Their forms are very 
irregular, and their duration exceedingly variable; 
and they are usually surround :d by a penumbra, or 



THE SUN. 



103 



imperfect shadow. They are almost always com- 
prised within a zone of which the extent varies, north 
and south of the solar equator; most of them, however, 
falling within the width of 60°. The appearance of 
these spots, when viewed through a good telescope, is 
often similar to that in the annexed diagram. 

Fig. 23. 




Suppose a b c, Fig. 23, the sun, and d e f spots 
upon its disk; the interiour portions of which are 
black, while the surrounding penumbra is of a lighter 
shade. 

Much ingenuity has been expended in endeavours 
to explain the nature and character of these spots. 
Some have imagined that the sun, from which a great 
quantity of heat and of light are constantly emanating, 
is a body in constant combustion, and that the dark 
spots are masses of scoria, or dross, floating upon its 
surface. The bright spots, on the contrary, have been 
attributed to a species of volcanick eruption of this 
mass of melted matter. The objection to this opinion 
is, that it does not furnish an explanation of the ap- 
pearances presented by the sun; and consequently it 
has not obtained the assent of astronomers. The 



104 DISTANCES, ETC. OF THE PLANETS. 

theory which, at the present day, is received with the 
most favour, is that which considers the sun as com- 
posed of a dark and solid nucleus, surrounded by two 
atmospheres, the one dark and the other luminous. 
Upon this hypothesis the appearance of the spots is 
explained by supposing changes produced in the atmos- 
pheres, which expose to view the dark body of the 
sun. The penumbra is the dark atmosphere, the fis- 
sure in which is supposed to be less than that in the 
luminous one, and which consequently remains visible 
around the borders of the opening which exhibits to us 
a portion of the sums nucleus. 

This opinion, however whimsical it may appear, 
has the advantage, at least, of explaining perfectly all 
the phenomena; and it acquires a high degree of pro- 
bability when we consider that the incandescent matter 
of the sun can be neither a solid nor a liquid, but is, 
necessarily, a gas. 

Luminous rays, emanating from a solid or liquid 
sphere, highly heated, manifest the property of polari- 
zation, while those escaping from incandescent gas 
are deprived of this property. It is the application of 
this principle to the experiments made upon the rays 
of the sun, which has conducted to the conclusion 
above stated, respecting one of the physical condirions 
of the sun. 

These experiments are made by means of a very 
ingenius instrument, the principle of which rests upon 
the properties of polarized light. This instrument is 
a species of telescope, so arranged as to give two 
coloured images of the sun, when pointed at that body. 
A very simple mechanism enables the observer to sepa- 
rate these images, entirely, or to superimpose one upon 
the other, either wholly or in part, at his pleasure. 
This instrument seems to determine that the light from 
the borders of the sun is equally as intense as that 
from the centre; for, if we superimpose the two images 



THE SUV. 105 

of the sun in such a manner that the border of one 
image coincides with the centre of the other, a per- 
fectly white light will be produced at this point of co- 
incidence. From this fact it follows: 1st, that the 
borders of the sun yield light equally as intense as 
that from its centre; and, 2d, that the colours of the 
two images, produced by the telescope, are comple- 
mentary one of the other. 

But from this fact, namely, that the light from the 
borders of the sun is equally vivid as that from its 
centre, there results another consequence. It is, that 
the sun has no atmosphere outside of its luminous 
covering, for, if it were otherwise, the light from the 
borders of the sun, having a thicker layer of this 
atmosphere to traverse, would thereby become more 
enfeebled. 

What is the nature of the sun's light? This ques- 
tion has long divided philosophers. One class of these, 
supported by the authority of Newton, supposed that 
the sun, as well as all luminous bodies, has the pro- 
perty of launching forth, with prodigious velocity, 
very minute particles of its own substance; this is 
known as the system of emission. Others, on the 
contrary, suppose that the phenomenon of light is pro- 
duced by the vibrations of a fluid called ether, which 
ry where distributed throughout nature, and is 
put in motion by the presence of luminous bodies: 
this is known as the system of vibrations or undula- 
; and it is the one now generally received; for 
no person can understand how a body can continually 
emit portions of its particles, and yet lose none either 
of its volume or its light. But the most serious and 
fatal objection to the system of emission is that it does 
not, in the present state of our knowledge, furnish an 
explanation of all the conditions of the case; while the 
other theory has all probabilities in its favour, particu- 
larly since recent discoveries have shown the most 
9* 



106 DISTANCES, ETC. OP THE PLANETS. 

intimate relations between the cause which produces 
the electrical phenomena and those which result in the 
production of light. 

Pouillet, a French philosopher, has devised a very 
ingenious method of determining the temperature of 
luminous rays. His method is the following: Sup- 
pose, says he, a sphere of ice, with an opening suffi- 
cient for the introduction of a thermometer to its cen- 
tre; which instrument we will suppose to be stationary 
there, at zero. Then, if luminous rays be admitted 
through the opening, and suffered to fall upon the ther- 
mometer, the mercury of the instrument will ascend 
in the tube, and thus indicate the augmentation of heat 
received from the rays. Now, if we know the dis- 
tance of the thermometer from the luminous body, the 
proportion that the opening through which the rays 
are admitted bears to the entire circumference of the 
sphere, and the quantity, in degrees and parts, which 
the ascent of the mercury, in the instrument, indicates, 
we may then calculate the quantity of heat which is 
produced by the body from whence the luminous rays 
have proceeded. In this way, whatever may be the 
distance of the luminous body, provided that distance 
be known, the mean quantity of heat, acting upon the 
thermometer, is always easily ascertained. 

This philosopher found, by this method, that his 
thermometer, thus situated, never rose above seven 
and a half degrees, nor sank below six; which gave 
him a mean of about 1200° for the temperature of the 
solar rays. 

It has often been asked if the rays of light, of which 
the velocity is excessive, have an appreciable impul- 
sive force? Their velocity, we are able to demonstrate, 
is, in a right line, at a rate no less than 192,500 miles 
per second of time; but the most delicate experiments 
have not yet been able to detect, or render sensible, 
the least impulsive force, in these rays. The fact 



THE MOON- 107 

has, indeed, been assumed, for the purpose of sustain- 
ing a theory, with regard to comets, as the pupil will 
learn in the subsequent part of this volume; but we 
should ever carefully distinguish between mere theory, 
and demonstrative facts. 

THE MOON, ©. 

The moon, we have already seen, is, in volume, 
only about one fiftieth part so great as the earth, and 
its distance from us is about 240,000 miles; so that 
with an instrument which magnifies one thousand 
times we should see this planet the same in size and 
appearance, as if we were viewing it, with the naked 
eye, at the distance of 240 miles. 

The movements of the moon are exceedingly com- 
plicated; and they were, for a long time, the source of 
very great embarrassment to astronomers. The moon 
moves in an ellipse, of which the earth occupies one 
of the foci, or focuses; and it describes this ellipse in 
29 days, 12 hours, 44 m 2 s . The moon is, at the same 
time, borne along through space, by the earth, in its 
movement round the sun; and while this last consumes 
one whole year to accomplish a single revolution in 
its orbit, the moon, in the same time, traverses her 
orbit thirteen and a half times. The moon turns upon 
its axis in precisely the same time that it executes its 
revolution round the earth; for which reason it never 
presents to our view any other than the same portion 
of its surface. 

It is from the combination of its different motions, 
that arise the phases of the moon; in other words, the 
different aspects under which we see this planet, at 
the different periods of its course. To illustrate these, 
suppose, Fig. 24, S the sun, and T the earth, and we 
have there a view of the moon in its various phases, 
or appearances. When if is at A, and therefore in 



108 DISTANCES, ETC. OF THE PLANETS. 

conjunction with the sun, it presents to the earth the 
unenlightened half of its surface; and it has therefore 

Fig. 24. 



the appearance as shown at a. Arrived at B, after 
having run over one eighth part of its orbit, from the 
point of conjunction, it will present to the earth one 
quarter of its enlightened part, and will then appear 
as shown at b. At C it will have described one quar- 
ter of its orbit, and consequently will there exhibit to 
the earth one half of its illuminated surface, as seen 
at c. At D it will show more than half the illuminated 
portion, and will appear as at d; while at E the whole 
enlightened half becomes visible from the earth, the 
moon being then full, as shown at e. From the point 
E the enlightened, visible portion of the moon declines; 
and through the remaining half of its orbit this body 
presents the same phenomena as in the first half, ex- 
cept that they occur in an inverted order, as shown in 
the figure; the interiour circle of which exhibits the 
moon as it would appear to a spectator placed upon 
the sun, and the exteriour circle shows its several ap- 
pearances to a spectator upon the earth. 



THE MOON- 109 

Such are the different phases which the moon ex- 
hibits, all within the space of 29 days and a half. 
When it is full, that is, when it presents all its en- 
lightened side to the earth, as at E, we say it is then 
in opposition with the sun; when it is new, that is, 
when none of its enlightened surface is visible to the 
earth, as at A, it is invisible to us; and we then say it 
is in conjunction. These two points are called the 
syzygies. It is there that the eclipses of the sun and 
moon take place, as will be shown hereafter. Finally, 
the moon is in its first or its last quarter, when it 
shows to us one half of its enlightened part; and these 
positions have received the name of quadratures, 
while the points intermediate between the quadratures 
and the syzygies are designated octants. 

The movement of the moon is much more rapid 
than that of the sun. Indeed, the sun advances less 
than one degree each day, while the velocity of the 
moon is about thirteen times greater, from whence its 
return to the meridian is retarded 48 al 46 s each day. 
It is to the difference of the rapidity of these move- 
ments that is to be ascribed the return of the conjunc- 
tion after 29 days and a half. 

The plane of the orbit of the moon is inclined to 
the ecliptick by the mean quantity of 5° 8' 49"; and 
the points of the intersection of these planes are called 
nodes. One of these is the ascending node, &, when 
the moon advances towards the north; and the other 
descending, y, when it recedes towards the south pole. 
Incontestable facts, based upon the most exact obser- 
vations, prove to us that the nodes of the moon have a 
progressive motion towards the west, thus passing 
over the ecliptick in the direction contrary to the ap- 
parent movement of the sun, or in the direction of the 
daily motion from east to west. The node thus shifts 
its place westward about 19£° during the year, which 
makes about 1° for every nineteen days, or an entire 



110 DISTANCES, ETC. OF THE PLANETS. 

revolution of the heavens every eighteen years and 
a half. More exactly, the nodes retrograde 19°. 3286 
per year; and consequently accomplish the circuit of 
the ecliptick in 6788 days .54019. What is desig- 
nated the synodical revolution of the node is a period 
of 346 days .61963; that is to say, after this interval 
of time the sun arrives again at the mooir s node. As 
the sun moves in a direction contrary to the node, this 
luminary comes again to the node before it has fully 
completed the entire circuit of the heavens; which 
explains why this period is less than an entire year. 
The pupil has already learned that the moon executes 
its revolution in its orbit, and its rotation upon its axis 
in the same time, and that it should, therefore, and 
does, present to us constantly the same face. But, 
notwithstanding this, we perceive, by the observation 
of spots upon the moon, that it also shows, sometimes 
a little more than this, and sometimes a little less, upon 
one side and then the other, as if it had a slight 
swinging or balancing motion. This is called the 
mooir s Hbration; a term which expresses the motion 
which we have mentioned, and it well describes the 
appearance in question, although it must not be taken 
literally, for this apparent oscillation is the result of an 
optical illusion. 

Indeed, the movement of the moon, in its orbit, va- 
ries, according as it approaches the earth or elongates 
itself from it, while its movement of rotation is always 
uniform. From this it results that, during the moon'*s 
acceleration it shows, upon its eastern border, some 
portion of its surface, which was not before visible, 
while a corresponding portion, upon the west, disap- 
pears from view: and the same phenomenon, but re- 
versed, is produced during the period of the moon's 
retardation. This is called the moon's libration in 
longitude. 



THE MOON. Ill 

The libration i:i latitude, arises from the fact that 
the moon, presenting constantly the same hemisphere 
towards the centre of the earth, and the observer not 
being placed there, but upon the earth's surface, per- 
>. when the moon is in the horizon, a portion 
more, upon one side, and a corresponding quantity 
rjon the other. 

PHYSICAL CONSTITUTION OF THE MOON. 

The phenomena of the phases have proved to us 
that the moon is not, like the sun, luminous, of itself, 
but that it is an opaque body, which shines only by 
reflecting borrowed light There is, upon the unen- 
lightened portions of the moon's disk, a faint light 
perceptible, which has a slightly reddish tinge. This 
proceeds from the light which the earth reflects to the 
moon, and which must fall upon all parts of her hemis- 
phere which is turned towards us. 

When we observe the disk of the moon, with the 
naked eye, many irregularities are perceived upon it; 
the nature of which we are unable to determine. But 
upon aiding the view with a good telescope, we readily 
distinguish, soon after new moon, and upon those 
parts of this planet that are not then enlightened by 
the sun, a great number of luminous points, which 
continually augment, in size, in proportion as the rays 
of the sun come more directly upon the surface which 
they occupy. Upon the sides of these which are op- 
posite to the sun, there are projected dense shadows 
which, as the sun changes place will be observed al- 
to remain, as at first, upon the sides directly 
opposite to that luminary. These brilliant points are 
high mountains, which receive the 
rays of the sun before the less elevated portions of the 
surface are enlightened by it, just as the mountains 
of our earth do, at sunrise: and the dark portions, 



112 DISTANCES, ETC. OF THE PLANETS. 

clouded with shadows, arc the cavities, or valleys be- 
tween these mountains, which usually somewhat re- 
semble the craters of volcanoes. Geometry has sup- 
plied us with the means of measuring the heights of 
these mountains. They are found to be very elevated, 
for a globe no larger than the moon; although they 
are not so high as some of the peaks of the Himaleh 
mountains, in Asia. The shadows which these moun- 
tains project have enabled us to measure their height, 
as well as the depth of the valleys. It is to the pre- 
sence of these mountain peaks that is to be ascribed 
that irregular row of beautifully bright but detached 
spots, like silver points, which are sometimes seen, 
through the telescope, bordering the disk of the moon; 
that appearance being produced by the illumination of 
the tops of the mountains, by the sun r s rays, while 
the valleys, between these peaks, are still in darkness, 
those rays not having yet attained the base of the 
mountains, as they do during full moon. 

The moon has no atmosphere: or, at least, if it has 
one, it is so rare as not to differ essentially from a 
void, since it produces no refraction of the rays of 
light. This is demonstrated by the immersions of the 
stars. These bodies, whenever eclipsed, to us, by the 
moon, are invisible the exact periods of time which 
calculations require them to be; which could not be 
the case, if the moon had an atmosphere which re- 
fracted the rays of light while passing from these stars 
to us. 

The axis of the moon being almost perpendicular 
to the ecliptick, the sun never sensibly departs from 
the moon's equator, to move north and south, at diffe- 
rent periods of the year, as it does with us; and from 
this circumstance it is that the moon does not experi- 
ence the variety of seasons which we enjoy upon the 
earth. But, as the moon turns upon its axis only once 
during an entire revolution round the earth, in its 



THE MOOX. 113 

orbit, each of its days and each of its nights is about 
fifteen times twenty-four of our hours; and what is 
far more singular, still, is, that one half the moon is 
enlightened by the reflected light from the earth, during 
the absence of the sun, and consequently has no abso- 
lute night, while the other half has no light whatever, 
save that of the stars, for periods of about fifteen of 
our days. 

Lagrange has endeavoured to explain the cause of 
the moon's revolving in her orbit, and turning upon 
her axis, in precisely the same period of time. He 
supposed — and he extended this supposition to all the 
other satellites — that the face of the moon, which is 
turned towards us, is much elongated, or protruded 
from the centre, comparatively to the opposite part, 
and that this excess of matter, and of course of weight, 
causes the moon to remain with the same face towards 
the earth, in obedience to the attraction which our 
planet constantly exercises upon it. 

To a spectator placed upon the moon the earth would 
appear thirteen times larger than the moon appears to 
us; and it would, to such observer, present regular 
phases, as the moon does to us, as shown, Fig. 24, 
page 108. These would at all times be invisible to one 
half the moon; while from the centre of the other 
half, sight of the earth would never be lost. 

As the earth turns upon its axis the appearance that 
it would present to a spectator upon the moon would 
necessarily be very varied. The seas, the continents, 
the extensive forests, and the numerous islands would 
present portions of various magnitudes and of diffe- 
rent degrees of light; while the atmosphere, with its 
clouds, must continually modify these several tints. 

We have already seen that the sun is continually in 

the equator of the moon; and from this circumstance 

inhabitants there would not enjoy the same means that 

we do, of calculating time. We measure the year 

10 



114 DISTANCES, ETC. OP THE PLANETS. 

by the return of the equinoxes, while their days would 
be always equal. They would, however, be enabled 
to measure by observing the poles of our earth, one 
of which would become enlightened and the other 
obscure, at each return of the equinoxes. 

Philosophers have carefully endeavoured to ascer- 
tain what are the properties of the luminous rays 
which are reflected to us from the moon; but the most 
delicate experiments have wholly failed to disclose, in 
them, any heating, or chymical properties, whatever. 
On the contrary, these rays, when concentrated in the 
focus of the largest mirrors, produce no sensible heat. 
For this experiment a bent tube has been employed, 
the extremities of which were terminated, each by 
a ball, filled with air, one of which was transparent 
and the other blackened, the middle of the tube being 
occupied by a coloured liquid. In this instrument, 
when there is an absorption of heat, the black ball 
absorbs much more than the other, and the air which 
it contains, being thus expanded, forces the coloured 
liquid towards the opposite extremity. This instru- 
ment is so delicate that it readily indicates the change, 
in temperature, of one thousandth of a degree; and 
yet, in the experiments cited, it showed no change. 
We are justified, then, in the assertion that the light 
reflected to us, by the moon, has no sensible calorifick 
properties. That it has no chymical properties is 
equally well sustained. The hydro-chlorate of silver, 
a substance which is instantly blackened under the 
influence of the sun, has been exposed to its action 
without the least perceptible result. 

But, notwithstanding these facts, credulity has at- 
tached to the light of the moon, in almost all countries, 
peculiar and extraordinary influences over agricultural 
productions. In France the field cultivators apply to 
the moon a particular name, during the last of April 
and the early part of May, as descriptive of its per- 



THE MOON. 115 

nicious agency, at that period. It is then, say they, 
that the moon freezes the yet tender buds, and exer- 
. over all tender vegetation, a malign influence. 
Now, it is very easy to exculpate the moon from the 
charge of these misdeeds. The period of the year 
here fixed upon is one when, in France, where these 
notions prevail, the temperature is often not more than 
four, five or six degrees above freezing. Now it is 
well known that plants lose, during the night, by ra- 
diation, a portion of the heat which they have received 
during the day; and experiments prove that this loss 
amounts to seven or even eight degrees, when the 
nights are serene, and without clouds to counteract the 
effect of radiation. When clouds prevail, these ra- 
diate, in their turn, towards the earth, forming, thus, 
a species of screen by which the heat that is radiated 
from the earth is arrested, and prevented from esca- 
ping into the upper regions of the atmosphere. The 
temperature of the young plants, which was not more 
than from 4 to 5 degrees above freezing, during the 
day, would thus fall, during the night, by radiation, 
several degrees below the freezing point, and of course 
would become congealed. But, as this great amount 
of radiation could not take place except upon such 
nights as when the heavens were unclouded, and con- 
sequently, when the moon would be visible, effects 
which are only the ordinary and necessary result of 
the change of temperature which we have mentioned, 
have been ignorantly ascribed to the freezing influence 
of the rays of the moon. The faith in this errour 
has been much strengthened — perhaps, indeed, it was 
originated — by the success of the precautions which 
are often taken, by these people, against the supposed 
agency of the moon; but which are, in truth, pre- 
cautions against the effects of radiation. The gar- 
deners, in such cases as we have* stated, to guard the 
tender buds and shoots from the rays of the moon, 



116 DISTANCES, ETC. OP THE PLANETS 

during its supposed reign of malignity, cover them 
with straw, or other materials, which forms a screen, 
serving, like the clouds, to prevent radiation. In this 
way, while they save their plants, they suppose they 
have proved the moon's agency in their destruction. 

Equally fallacious and absurd are the important 
agencies which agricultural and pastoral people, in 
almost every country, even at this late day, ascribe 
to the moon, in determining the seed time, and the 
success or failure of their various crops, and the 
character of the healthy or diseased condition of the 
young of their domestick animals, &c. All this is no 
other than the remains of that ancient Astrology 
which so long held the world in supertitious dread of 
its secret influences; and it can only be removed by 
minute and accurate acquaintance with such laws of 
nature as investigations have laid open to our view. 

But it is not alone the present age which attributes 
noxious and peculiar influences to the moon. The 
ancients often did the same; and we have the authority 
of Pliny, Aristotle, and others, for saying that through 
the agency of the moon the bodies of oysters and other 
shellfish are increased and diminished; that the mass 
of man's blood is varied, in like manner, and by the 
same cause; that all animals, but particularly man, 
expire at the time of ebb tide; that the moon inclines 
us to repose, &c. ; with much more, equally ridiculous 
and unphilosophical; and we cite this here only to 
show in what a catalogue of ancient, ridiculous fol- 
lies our present superstitions, with regard to the moon, 
belong. Plutarch, too, pretends that the moon's light 
putrifies animal substances. Now, it is very true that 
if we place, in the open air, two pieces of meat, for 
instance, and one of these be exposed to the rays of 
the moon, while the other is guarded by a screen, or 
covering, the first of these will be much sooner at- 
tacked, by putrefaction, than the last; but here, as in 



THE MOON 117 

the case of the plants, men have attributed to the moon 
an effect which it has no agency in producing. Al- 
though the piece of meat exposed to the moon's rays 
will become first tainted, yet this is wholly owing to 
the fact that, having no shield to check the radiation 
of heat from it, it will become much colder, in the 
same period of time, than the other piece, and con- 
sequently will be more highly charged with humidity, 
by condensation of water from the atmosphere: and, 
because water is a powerful agent, in the decomposi- 
tion of animal matter, we often dry meats to preserve 
them. 

Another errour, no less ancient, nor less widely 
diffused among mankind, is that which attributes, to 
the phases of the moon, and to its passage through its 
phases, an influence upon atmospherick variations, and 
the changes of weather. This popular errour, which 
we find in the most ancient, no less than the most 
modern authors, is wholly without foundation. Aside 
from the fact that we can see no way in which the 
action of the moon could produce any such results, 
the most exact and careful observations, continued 
through long periods of time, give the most formal 
denial to this assumption. The changes of weather 
are not more frequent, at the passages of the moon 
from one quarter to another, than at any other epoch; 
indeed, if there is any difference — though no rule pre- 
vails, which is distinguishable — it would often appear 
to be in favour of the octants. 

What, then, it may be asked, can be the cause of 
an errour so long and so constantly accredited? There 
may be several; and the general tendency of the hu- 
man mind, when uninstructed, to give credence to 
omens, of all kinds, is probably one. This pre-con- 
ception has doubtless been strengthened by a want of 
impartial observations, and by the involuntary incli- 
nation of all theorists to register and remember only 
10* 



118 DISTANCES, ETC. OF THE PLANETS. 

such facts as seem favourable to their p re-con 
opinions, while they habitually omit all account of 
such as militate against them. Consequently, a change 
of weather happening at a new quarter of the moon, is 
noticed as a confirmation of the assumed theory, while 
twenty other changes of quarters, that are unaccom- 
panied by any corresponding changes of weather, are 
not even observed. Another cause is readily found 
in the fact that, as the moon changes once in seven 
days, no change of weather can take place at a longe 1 : 
time than three and a half days from a change of the 
moon, while most of these must needs occur at much 
less intervals: and those who are wont to ascribe 
storms and other atmospherick changes to the agencv 
of the moon, have never deemed this short lapse of 
time any objection to their favourite superstition. In 
defence of the errour here exposed, the name of Theo- 
phrastus, a Greek scholar, who lived before the chris- 
tian era, has been cited; but this is an authority of but 
little weight in science, and particularly in a qu: 
of this sort, that has been determined by exact obser- 
vations. And even the authority itself involves a con- 
tradiction. Theophrastus states that the new moon 
brings stormy weather, the full moon fair; and that 
the weather changes at each quarter. Now if, at new 
moon, the weather is stormy, it will be fair at the 
second quarter, according to this authors position: 
and consequently stormy, again, at the full, in contra- 
diction with the previous part of the proposition. 

A learned modern, who has written a work in defence 
of the popular opinions, upon this subject, has sought 
to support them by scientinek considerations; but he 
has fallen into the grossest errours. and onlv obtained 
the results he sought by including a greater or less 
number of days in his observations, according as he 
had need of more or less of atmospherick changes. 



THE MOON. 119 

It has been often suggested, by scientifick men, that 
aerolites may have been projected from the moon; and 
among other considerations, in support of this sugges- 
tion, it has been urged that observations render it prob- 
able that volcanoes are in active operation in the moon. 
We may here remark that the presence, at different 
intervals of time, upon the obscure surface of the 
moon, of brilliant, shining points, and the forms of vol- 
canick craters which almost all the valleys of the 
moon appear to assume, are not sufficient evidence of 
volcanoes in the moon to establish their existence there. 
It is very true, however, that, the existence of such 
volcanoes once admitted, stones may be ejected by 
them with a force sufficient to carry them beyond the 
distance where they would cease to gravitate to the 
moon. Calculations have shown that, to accomplish 
this, such stones need only to issue from the crater of 
a lunar volcano, with a velocity equal to five and a 
half times that of a cannon ball; and volcanoes, upon 
the earth, have sometimes ejected stones with a greater 
velocity than this. Yet we must bear in mind that the 
resistance of our atmosphere, and the very great at- 
traction of the earth over that of the moon, would pre- 
vent such stones from being carried so far as not again 
to fall back to the earth. 

We will now proceed to examine the different hy- 
potheses by which this astonishing phenomenon, of the 
fall of meteorick stones, has been attempted to be ex- 
plained. To do this we must first enumerate the gen- 
eral circumstances which observation has made known 
in relation to these stones; all of which must, of course, 
be fully explained by any hypothesis, as the condition 
of its admissibility. 

Aerolites are ordinarily portions of a species of fla- 
ming meteor, sometimes designated globes of fire, 
which pass very rapidly through the air, at various 
heights above the earth, and in a direction nearly par- 



120 DISTANCES. ETC OF TUT PLANETS. 

allel with its surface. These, in some part of their 
flight, usually burst, or explode, and the fragments 
in such cases, fall to the ground. These are all 
composed of the same chymical properties, and very 
nearly iii ih? same proportions. They all contain 
much silex and iron, wil ; ;a, sulphur, nickel, 

manganese, and chrome. One of th h fell 

near Alais, in France, contained 
carbon; but perhaps those which have fallen elsewhere 
have originally contained it also, and lost it, in travers- 
ing our atmosphere: for these stones experience, du- 
ring their flaming course, so high a degree of heat that 
a great part of all the volatile principles which enter 
into their primitive composition must be evaporated. 
One important fact requires here to be mentioned, 
namely, that iron and nickel occur in these stones, in 
a metallick state, which is not the case in any of the 
mineral aggregations which we find upon the surface 
of the earth. It is certain, also, that these stones do 
not, naturally, pertain to the earth's surface; all which 
are known having fallen from the air. 

Such are the facts: for the explanation of 
eral systems have been propc : may be re- 

duced to the three following hypotheses: 

1st That aerolites, like rain and hail, are actual 
meteors, which are formed, by in the at- 

mosphere. 

2d. That they are either fra f planets v 

have been shattered, or small, entire planets which, in 
revolving in space, have been attracted into our at- 
mosphere, and, losing, gradually, their velocity, by 
the resistance c. have finally i 

to the earth. 

3d. Laplace. .jrof the ?-Iecanique Celeste, 

has suggested that aerolites may owe their origin to 
the eruptions ol lunar volcanoes, which may have 
ejected these stones with such force and velocity as to 



TITE MOON. 121 

carry them beyond the sphere of the moon's attrac- 
tion, and thus cause them to become new satellites of 
the earth, but of so small a mass as to be subject to 
very great perturbations. If, after having circulated 
for an indefinite period, in the regions of space, these 
small bodies be finally drawn within the atmosphere of 
the earth, the result, as in the preceding case, would 
precipitate them upon the earth. 

Of these three hypotheses, the first, which, without 
examination, appears the most simple and the most 
natural, is nevertheless the most improbable one of the 
three, and will not sustain a moment's examination. 

In order that these stones might be formed, by aggre- 
gation, in the atmosphere, it is necessary that the ele- 
ments of which they are composed should severally 
exist there. Rain and hail are, indeed, formed there, 
but their constituent is water, which is always present, 
in the air, in a state of vapour, and cold, only, is ne- 
cessary to condense it into drops, or a still greater de- 
gree of cold to form it into ice, or hail. But the most 
exact analysis of the atmosphere has not disclosed, 
therein, any of the constituent principles of meteorick 
stones: neither sulphur, magnesia, silex, nickel or iron. 
Nor have we any proof that oxygen and azote, of 
which the atmosphere is composed, have any power to 
dissolve these substances. But the objection to this 
has been that all these experiments have been made 
upon portions of air taken at the surface of the earth; 
while there may be, in the upper regions of the air, gases 
capable of holding in solution those metals and earths 
of which aerolites are formed. But the response to this 
is, that the air obtained upon the highest parts of 
mountains to which man has been able to ascend has 
been submitted to analysis, and it is found to be abso- 
lutely the same, in its composition, as that at the sur- 
face of the earth. Thi3 result was easily foreseen, 
because it is a general law of staticks, applied to the 



122 DISTANCES, ETC. OP THE PLANETS. 

gases, that they extend themselves, in time, through 
all space which is open to them; and that, when seve- 
ral of these, of different natures or gravities are super- 
imposed, the whole so combine as to form one homo- 
geneous mass. If, then, there exists, in the upper re- 
gions of the atmosphere, any gas, capable of holding 
in solution the earths or metals which enter into the 
composition cf aerolites, we should necessarily find 
something of it in the air at the surface of the earth; 
and, as we do not, the assumption that there is such 
gas there, is considered to be without foundation. 

To this first impossibility are joined several others. 
Should it be admitted that the principal constituents of 
aerolites really exist in the atmosphere, at all heights, 
and that they escape detection, by analysis, because the 
quantity is so small, still it would be necessary to ex- 
plain how it is possible for materials so scanty, and so 
widely diffused through the air, to be so suddenly ag- 
gregated into masses constituting stones of several 
hundred pounds weight, like those preserved at En- 
sisheim, in France: or of three or four thousand 
stones, of various sizes, like those which have been 
detached from a single meteor, in its course through 
the air. It is necessary, also, to assign the cause 
which thus unites these scattered particles, and forms 
them into such huge masses. This is not affinity, for 
the elements which compose aerolites are not combi- 
ned, but simply agglomerated and retained together, 
in juxtaposition. And yet, if these particles had not 
been subject to the action of any force, they would fall 
to the earth, individually, as fast as formed, instead 
of coming down, as they do, in such ponderous masses. 
It is in vain to object, here, that they may be sustained 
more or less time in the air by some cause analogous 
to that which, according to the ingenious opinion of 
Volta, balances hailstones between the clouds, in such 
a manner as gives them time to augment in bulk, by 



THE MOON. 123 

successive layers of ice; for these hailstones have ne- 
ver been known to reach so disproportionate a mass 
oral hundred pounds weight, although water, of 
which they are composed, is much more abundant, in 
the air, than, even by this hypothesis, the constituent 
elements of aerolites can be supposed to be. Farther- 
more, in the opinion of Volta, the suspension of hail- 
stones, in the air, is attributable to the reciprocal ac- 
tion of electrical clouds; and this cause cannot be 
adapted to the facts in cases of aerolites, as they often 
fall at times when the heavens are wholly um-louded 
and serene. Finally, if aerolites were formed in the 
atmosphere, like rain and hail, they would, like these 
last, in obedience to the laws of gravity, fall 'in per- 
pendicular lines to the earth; or they would deviate 
no more from such lines, than might arise from the 
influence of the winds. But this is not so; for the 
aerolites have, in their fall, a very rapid horizontal 
motion, in some instances nearly or quite equal, in ve- 
locity, to the motion of our earth in its orbit. This 
fact, alone, is sufficient to show the impossibility of 
the formation of meteorick stones in the atmosphere; 
if, indeed, the chymical objections which we have al- 
ready cited, have not fully established this fact. 

The second hypothesis which has been formed, upon 
the origin of these masses, is much more probable. 
So small planets have recently been discovered, that 
some degree of probability seems thus given to the exis- 
tence of still smaller ones, of which our meteorick stones 
may have formed portions. These small planets, en- 
tering into our atmosphere, and there losing, little by 
little, their original velocity, would thus be brought, at 
last, to the earth. Before this could take place, how- 
ever, the compression of the air, before the moving 
body, would undoubtedly be sufficient greatly to heat 
the stony mass; so that the combustible materials it 
might contain would be lighted into flame, as Wf 



121 DISTANCE?. ETC. OF THE PLANETS. 

actually take place. This hypothesis, then, fully ac- 
counts for all the circumstances attending the fall of 
these ineteorick bodies; but it in r lains their 

identity of composition, unlet ;me that ail the 

planets so small as to belong to this class, are abso- 
lutely ol the same nature and composition; of the same 
elements, in the same proportions; which assuption is 
not warranted by any thing we know upon the earth; 
and which, applied to the other bodies of our sole 
tern, is exceedingly improbable. There is. aJe 
this hypothesis, something at open war with all we 
know, and all that we have reason to believe, in nature; 
for it involves a degree of imperfection in the mecha- 
nism of the Universe, which implies its destruction by 
age, unless that catastrophe shall be prevented by oc- 
■nal repai- 
But the identity of the chymical composition of these 
aerolites finds its ready explanation in the third hy- 
pothesis, namely, that which supposes these stones the 
product of lunar volcanoes: for then it is sufficient to 
suppose, either that all these volcanoes emit only such 
substances: or that the stones are the production of one 
or more particular ones, which al : their con- 

tents with sufficient force to throw them beyond the 
sphere of the moon's attraction. That degree of force, 
as calculation has shown, is not very great, because 
the moon is not surroimded by an atmosphere which 
would resist their flight But. as we have before ob- 
served, if the existence of lunar volcanoes is rendered 
probable, by astronomical observation by no 

means demonstrated. Yet if the existence of these 
volcanoes be once admitted, the explanation of the phe- 
nomena then becomes one strictly mechanical. It is 
easy to conceive, between the earth and the moon, a 
line which marks that division in space where the at- 
traction of the earth and that of the moon is equal. 
This, of course is much nearer the moon than the 



DISTANCES, ETC. OF THE PLANETS. 125 

earth, because the mass of the moon is so much less 
than that of the earth. When the meteorick stones, 
issuing from the lunar volcano, have once passed this 
line, which may take place in a variety of directions, 
they must instantly become satellites of the earth: but 
they would be satellites which would experience enor- 
mous disturbances, by reason of the smallness of their 
masses, relatively to those of the earth, the moon and 
the sun, by all of which they must be powerfully at- 
tracted. When, by reason of these disturbances, oneof 
these stones shall have been brought within the atmos- 
phere of the earth, the resistance of that atmosphere 
would lessen its velocity more and more, until, as in 
the preceding case, the body would finally fall to the 
earth. 

We have thus submitted the facts which support the 
hypothesis of aerolites 7 emanating from the moon, 
and which seem to render that the most probable of 
any yet suggested; since it is the only one which satis- 
rily accounts for all the phenomena coupled with 
this interesting subject. But we must not forget that, 
as we have before stated, the existence of volcanoes in 
the moon is by no means established; nor should we 
overlook, here, the fact that the transfer of matter from 
one planet to another, which this hypothesis involves, 
is, no less than tfye preceding, obnoxious to the charge 
of involving, also, that imperfection of the mechanism 
of the universe to which we have before alluded. 
11 



126 OF THE PLANETS. 

CHAPTER SIXTH. 
IXFERIOUR PLANETS. 

MERCURY, *. 

Mercury is the nearest planet to the sun. It is visi- 
ble, at night, after the setting of the sun, in the west- 
ern part of the heavens, under the form of a small, 
but very brilliant disk, which, at first, difficult to dis- 
tinguish, by reason of the twilight, becomes more and 
more visible, in proportion as the planet's distance 
from the sun increases, until, reaching its utmost dis- 
tance from that luminary, it becomes, for sometime ap- 
parently stationary. This first part of its course is 
direct, like that of all the planets. It soon, however, 
again approximates the sun, and finally becomes lost 
in its rays. Soon after this disappearance in the icest % 
it reappears in the east, rising before the sun. Here, 
of course, it is on the opposite side of the sun, from its 
Ion when seen in the west: and, as before, it gra- 
dually increases its distance from the sun, until, reach- 
ing its utmost limit, it first becomes stationary, in 
appearance, and then slowly plunges itself again into 
the rays of the sun, and disappears — again to present 
itself in the west, as before. 

The short duration of the appearance of Mercury 
arises from its vicinity to the sun, from which its an- 
gular distance is never more than about 29 z . nor less 
than about 16 °. The mean distance of Mercury, 
from the sun, is about 37,000,000 of miles. Its ap- 
parent diameter is about 7 , and its real diameter is 
very nearly two fifths of that of the earth, or about 
3,200 miles. It turns upon its axis in *24 b 5~ 3': while 
it occupies 57- Co- 14- 30 s in accomplishing a revolu- 
tion in its orbit. This orbit, which is wholly enclosed 
within that of the earth, is in the form of a very ec- 



MERCURY. 127 

centrick ellipse, and greatly inclined to the plane of 
the equator of Mercury; and making, with the plane 
of the ecliptick, an angle of about 7°. 

While Mercury is wholly immersed in the rays of 
the sun, it is sometimes seen passing across the disk of 
the sun, where it has the appearance of a small, round, 
black spot. This is known to be Mercury, because 
the position, the diameter and the motion are all those 
which belong to that planet. This passage across the 
sun is called the transit of Mercury. This does not 
take place at every revolution, by reason 01 the incli- 
nation of Mercury's orbit to the plane of theecliptick; 
and we can therefore only see this planet upon the 
disk of the sun when it is at its point of intersection 
with the ecliptick, and when a line joining the centres 
of the sun and Mercury would, if prolonged, pass 
through the centre of the earth. The small size of 
this planet, its distance from the earth, and its close 
proximity to the sun, often prevent us from witnessing 
these transits, which take place, regularly, at periods 
of 6, 7, 13, 46 and 263 years. 

PHYSICAL CONSTITUTION OF MERCURY. 

Mercury is perfectly spherical, in its form; and, like 
all the planets, it borrows its light from the sun. This 
is fully proved by its passages across the sun's disk, 
when, having no rays of the sun falling upon the he- 
misphere which it presents to us, its appearance, as 
stated above, is that of a black spot. The same is also 
proved by the observation of the phases which Mercury 
presents, and which, with the assistance of the tele- 
scope, we are enabled to follow, through all their 
changes, like those of our moon. 

The employment of this instrument has also shown 
us that one of the extremities of the crescent of Mer- 
cury is truncated, or apparently cut off; and it is this 



128 OP THE PLANETS. 

circumstance which has furnished astronomers the 
means of determining the period of this planet's rota- 
tion upon its axis; for its disk is not, like the moon, 
for instance, furnished with fixed spots, easily recog- 
nised. The appearance in question is an effect of the 
asperities with which the surface of the planet is 
doubtless more or less covered, and which mask from 
us, in a particular position, some of that portion en- 
lightened by the sun. 

We know that Mercury is enveloped by an ex- 
tremely dense atmosphere. Its motion in its orbit, 
through space, is more rapid than that of the other 
planets, because it is nearer the sun. The sun, if 
viewed from Mercury, would appear three times 
greater than it does to us; and Newton has calculated 
that this body emits to Mercury an amount of heat 
seven times greater than that experienced within the 
torrid zone of the earth. But we are not warranted 
in concluding that this planet really experiences so 
elevated a temperature; for we are not yet sufficiently 
instructed in the producing causes of heat to justify 
us in doing so; since it may very well be that the ac- 
tion of luminous rays is modified by the nature of the 
constituent elements of the different planets. 

VENUS, 5. 

Venus... is the most beautiful of the planets; which 
explains why it has received the name it bears. Like 
mercury, Venus shows itself sometimes in the morn- 
ing and sometimes at evening; and it is called the 
morning or evening star, according as it is visible 
before the rising or after the setting of the sun. Some 
days after its conjunction with the sun, Venus is seen, 
at first, in the morning, west of the sun — and of course 
rising before it— in the form of a beautiful crescent, 
of which the convexity is turned towards the sun. 



VENUS. 129 

It is then moving west, among the stars, and as it ad- 
vances its velocity decreases, while its crescent con- 
tinually augments in size, until the planet reaches its 
greatest angular distance, and becomes stationary, 
when the crescent forms a half circle. From this 
point the planet returns eastward towards the sun, 
with a gradually increasing velocity, until it is lost in 
the solar rays. After this it is next seen at evening, 
east of the sun. Its form, then, is entirely round, and 
its apparent size is greatly diminished. Its motion, 
then, is to the east; while its diameter constantly in- 
creases, and its form gradually changes from round 
to the crescent, until the planet has reached its great- 
est eastern limit, when its appearance has again be- 
come that of a half circle. From this position it 
again withdraws towards the west, gradually approach- 
ing the sun, until it is once more lost in its beams. 

The distance of Venus from the earth, like that of 
mercury, is very variable; as is evident, in the case 
of both planets, by the apparent variations of their 
diameters. The mean distance of Venus from the 
sun is 68,000,000 of miles; and its apparent diame- 
ter is very variable. This planet accomplishes its ro- 
tation upon its axis in 23 h 21 m 19 s ; and its revolution 
in its orbit, in 224 d 16 h 41 m . Its orbit is inclined 3° 
24' to the plane of the ecliptick, and is, like that of 
mercury, always enclosed within that of the earth. 
Venus, like mercury, sometimes passes across the 
sun's disk, and like that planet, Venus is seen there as 
a black spot. The phenomenon of these passages, in 
the case of Venus, is very rare, yet astronomers have 
profited by their occurrence to measure the sun's dis- 
tance with accuracy. The pupil has already seen 
that in this way the parallax of the sun has been de- 
termined to about one tenth of a second. 
11* 



130 OF THE PLANETS. 

PHYSICAL CONSTITUTION OF VENUS. 

When this planet is seen crossing the disk of the 
sun, it appears, like mercury, as a small round, black 
spot. We know, then, that its figure is spherical, and 
that its light is borrowed from the sun — which, indeed, 
we were authorized to infer from the appearances of 
its phases. The duration of the revolution of Venus 
upon its axis, like that of mercury, is determined by 
the asperities which this planet bears upon its surface, 
and which, intercepting the reflected light, give a 
truncated appearance to the extremities of the cres- 
cent. By carefully noting the time which elapses be- 
tween any two successive returns of this appearance, 
the time of revolution of the planet is disclosed. 
Venus is enveloped in an atmosphere. This fact was 
determined by a German astronomer, through the 
assistance of the laws of opticks; and he proved that 
the enlightened portion of the planet is greater than it 
could be, without the aid of the refraction which this 
atmosphere produces. 

Although very nearly as large as the earth, Venus 
moves with more rapidity, in its orbit, because it is 
much nearer the sun. From Venus the sun would 
appear almost twice as large as it does to us; and 
mercury is to Venus, morning and evening star, as 
Venus is to the earth. 

The axis of Venus is inclined to the plane of its 
orbit 75°; or 514° more than the axis of the earth to 
the ecliptick. The axis of Venus is so inclined that 
the northern regions of that planet have summer in 
the same signs of the zodiack in which we experience 
our winter; while our summer signs are those of win- 
ter to Venus. As the greatest declination of the sun, 
upon either side of the equator of Venus, is 75°, its 
tropicks, of course, are within 15° of its poles; and 
consequently its polar circles are this distance from 



MARS. 131 

its equator. This planet has, then, two summers and 
two winters in each of its annual revolutions. 

Careful and multiplied observations have been made, 
by astronomers, to determine if either mercury or 
Venus has any satellites; but none have been disco- 
vered; and these accompaniments appear to pertain 
only to some of the superiour planets, and to the earth. 

SUPERIOUR PLANETS. 

The two planets of which we have just treated are 
called inferiour planets, because they are, as we have 
already seen, less removed from the sun than is the 
earth; while those which are now to occupy our at- 
tention have been named superiour planets, from the 
fact that their orbits are outside that of the earth; in 
other words, that these planets are all more distant 
from the sun than the one upon which we reside. 

mars, $ . 

This planet comes next to our earth, in the order of 
distances from the sun. It appears to move among 
the stars, from west to east, round the earth; but its 
motion is subject to many irregularities. When first 
visible, in the east, before the sun, in the morning, its 
motion from that planet is very rapid: this, however, is 
gradually diminished, and finally wholly ceases, at 137°. 
The planet next resumes a direct motion, which car- 
ries it into opposition with the sun. Its rapidity again 
diminishes, gradually, and it seems to retrograde until 
it appears to have passed the sun 137°. Then the 
motion again becomes direct, and the planet is again 
lost in the rays of the sun. 

The mean distance of Mars from the sun is 
142,000,000 of miles. As its distance from the earth 
is very variable, this variation is manifested by the 



132 OF THE PLANETS. 

apparent dimension of its diameter, which constantly 
changes with the change of distance. Careful obser- 
vation of the spots which its disk presents has taught 
us that Mars turns upon its axis in 24 h 39 ra 22 s . It 
moves in a very eccentrick ellipse, in which it accom- 
plishes its revolution in 68G d 23 h 30 ra 42 s . 4. Its axis 
is inclined 61° 33' to its orbit; and the inclination of 
its orbit to the ecliptick is 1° 51' 1". The planet's 
equatorial diameter is, to its polar diameter, in the 
proportion of 16 to 15. 

Mars experiences, in its revolution in its orbit, great 
variations of distance from the sun: sometimes appear- 
ing near that body, and at others far from it: some- 
times it rises while the sun is setting, and sometimes 
it sets at sunrise. Its distance from the earth, also, 
varies greatly; but most in its conjunctions, and least 
in its oppositions. Like mercury and venus, Mars 
presents the phenomena of phases; but the truncated 
appearance of the crescents of these two planets is 
not distinguishable in Mars. 

PHYSICAL CONSTITUTION OF MARS. 

Viewed through a telescope this planet presents a 
round disk; and its surface exhibits great diversity of 
light and shade. Its phases prove to us that this planet 
shines by means of borrowed light, which is furnished 
by the sun. Spots have been identified upon the sur- 
face of Mars, by which the duration of its revolution 
upon its axis has been determined. The light which 
Mars reflects to us is of a dark red colour; which ap- 
pearance is attributed to the atmosphere by which the 
planet is surrounded, and which is so high and so 
dense that when the planet passes nearly in a line be- 
tween the earth and a fixed star, such star has been 
observed to change colour, become dim, and some- 
times wholly to disappear, although at a considerable 
•distance from the body of the planet. 



MARS. 133 

Aside from the spots which have served to deter- 
mine the rotary motion of Mars, several astronomers 
have remarked that a segment of its globe, towards 
the south pole, has a light so superiour to that of the 
rest of the disk, that it appears like a portion of some 
other globe, more strongly illuminated. Maraldi in- 
forms us that this brilliant spot had been observed for 
sixty years, and that it is the most permanent of all 
those upon Mars. Parts of this planet, as before 
stated, are more brilliant than the rest; while the 
darker portions are subject to great changes, and some- 
times disappear. A similar light to that we have men- 
tioned at the south pole, has often been observed, also, 
at the north pole. These observations were confirmed 
by Herschel, the elder. According to this astronomer 
the analogy between Mars and the earth is by far the 
greatest in the whole solar system. Their diurnal 
motion is nearly the same; the obliquity of their re- 
spective eclipticks, on which the seasons depend, not 
very different; while, of all the superiour planets, the 
distance of Mars from the sun is, of course, far the 
nearest to that of the earth: nor will the length of its 
year appear very different from ours, when compared 
to the surprising duration of the years of jupiter, 
saturn, or uranus. If, then, we find that the globe 
which we inhabit has its polar regions frozen, and 
covered with mountains of ice and snow, that only 
partly melt when alternately exposed to the sun, we 
may well be permitted to surmise that the same causes 
may probably have the same effects on the globe of 
Mars; that the bright polar spots are owing to the 
vivid reflection of light from its frozen regions; and 
that the reduction of those spots is to be ascribed to 
their being exposed to the sun. The spot at the south 
pole was extremely large in 1781, as, upon this hy- 
pothesis, it should have been, because that pole had 
then but recently emerged from a night of twelve 



134 OP THE PLANETS. 

months, during all which time it was deprived of the 
heat of the sun; but in 1783 it was considerably- 
smaller than before, and it continually decreased from 
the 20th of May to the middle of September, when 
its size became stationary. During this last period, 
the south pole had already been more than eight 
months enjoying the benefit of summer, and it still 
continued to receive the sun's rays; though towards 
the close of the period these fell in so oblique a direc- 
tion as to impart but little heat to the surface. On 
the other hand, in 1781, the north polar spot, which 
had then been its twelve months in the beams of the 
sun, and was but just returning into darkness, appeared 
small, though doubtless it was increasing in magnitude. 
Its not being visible in 1783 was owing to the position 
of the axis, by which it was removed out of sight. 

There is another consideration which strongly con- 
firms the hypothesis that the brilliant spots about the 
poles of Mars are produced by the presence of snow 
and ice, which is, that the axis of this planet being 
inclined to its orbit 61° 33', the variations of the sea- 
sons cannot be very great; and this constancy of each 
parallel of latitude in preserving nearly the same tem- 
perature is regarded as favourable to the formation of 
ice. 

The sun dispenses, to Mars, only about one third as 
much light as is enjoyed by the earth; and yet it has 
no satellite, or moon. Perhaps this circumstance may 
be compensated for by the height and density of its 
atmosphere, which we have seen to be very consi- 
derable. 

OF THE FOUR TELESCOPICK PLANETS. 

These planets, which are placed, in the arrange- 
ment of the solar system, between mars and jupiter, 
were ail discovered within the present century. This 



JUNO, CERES, PALLAS. 135 

circumstance, coupled with their small size, and great 
distance from us, accounts for the fact that they are 
very little known. 

JUNO, §. 

This planet was discovered by Professor Harding, 
of Gottingen, on the first of September, 1803; and 
it has, according to Schrceter, a diameter of 1,138 
miles. It employs 4 years and 128 days to accomplish 
its revolution round the sun, in an orbit inclined to 
the ecliptick 13° .05. Its distance from the sun is 
about 256,000,000 of miles. 

CERES, "J. 

Of the four telescopick planets, Ceres was the first 
discovered The discovery was made by Piazza, at 
Palermo, on the first day of January, 1801. Its 
diameter has been stated at 140 miles, by some, while 
others have fixed it as high as between eleven and 
twelve hundred miles. It is manifestly not known. 
It makes its revolution round the sun in 4 years 220 
days, moving in an orbit of which the plane makes 
an angle of 10° 37' 25" with the ecliptick. Its dis- 
tance from the sun is about 264,000,000 of miles. 
The appearance of this planet is that of a nebulous 
star, or one surrounded by a kind of haze, continually 
changing in its appearance, which has led Herschel to 
suppose the planet to possess an atmosphere. 

PALLAS, $. 

This member of our solar system was discovered 
by Dr. Olbers, of Bremen, on the 28th of March, 
1802. Herschel gave to this planet a diameter of 
about 134 miles, while Schrceter fixed it at 14 times 



136 OF THE PLANETS 

this sum — sufficient evidence that nothing is known 
upon the subject. Its orbit, extremely elongated, has 
a greater inclination to the ecliptick than that of any 
other belonging to the solar system, being no less than 
34° 37' 30'. It revolves in its orbit round the sun in 
the period of 4 years, 7 months and 11 days; at 
the distance ot about 268,000,000 of miles from that 
body. Pallas has a whitish colour, and is never very 
distinct, even when view T ed with powerful telescopes. 

VESTA, g. 

Vesta was discovered by one of the scholars of Dr. 
Olbers, on the 29th of March, 1807. It describes, in 
three years, 66 days and 4 hours, its orbit, which ap- 
pears very irregular, and which is inclined 7° 8' 9" to 
the ecliptick. This small planet is but very imper- 
fectly known. Very powerful telescopes have been 
applied to it, without disclosing any disk; for it ap- 
peared, in them, like a brilliant point. Its distance 
from the sun is conjectured to be about 225,000,000 
of miles. 

Although our knowledge respecting these four 
planets is very imperfect, we are nevertheless enabled 
to state that they are extremely small, relatively to 
those which are near them, and to their great distance 
from the sun. Another anomaly which they present 
is, that they deviate much from the zodiack, the com- 
mon highway of the other planets. From these cir- 
cumstances the opinion has been ventured that these 
four small planets are no other than the fragments of 
one much larger which, revolving between mars and 
jupiter, by some internal convulsion, has been me- 
chanically broken into four parts. This opinion has 
been supposed to derive farther support from the fact 
that appearances have favoured the opinion that these 
planets are not round; and also, from the circumstance 



JUPITER. 137 

that their orbits are interlaced in such a manner as to 
cause all these planets to return to one and the same 
point during their revolutions; a fact which the laws 
of mechanicks show would be necessary to them, in 
the hypothesis in question: for, according to these 
laws, if a revolving planet were rent into two or more 
pieces, by internal violence, each fragment, after 
having described a new orbit, must return to the point 
where the disrupture took place. But, although these 
facts yield support, indeed, to the theory in question, 
yet they by no means confirm it: on the contrary it 
is so at variance with all we know in nature, that the 
strongest confirmation is requisite to secure to so sin- 
gular a position that credence which belongs to scien- 
tifick truth. 

The small size of these planets induced Herschel to 
deny to them the name of planets, and he therefore 
proposed to bestow upon them that of Asteroids. This 
was deemed an improper distinction, since these 
planets are not smaller, in comparison with mercury, 
than mercury is, in comparison with jupiter; and 
astronomers, therefore, when they have occasion to 
refer to these bodies, collectively, reject this name, 
and designate them telescopick planets, in reference to 
the difficulty of getting sight of them, as well from 
their small size, as, also, from their great distance. 



CHAPTER SEVENTH. 



JUPITER, 21, AND ITS SATELLITES. 

Jupiter is the largest of the planets; and, with the 
exception of venus, is the most brilliant. This body 
is 1470 times larger than our earth, and it is only by 
reason of its vast distance from us that it appears so 
small to us, in the heavens. Its revolution upon its 
12 



138 OP THi: PLANETS. 

axis is extremely rapid, and is completed in 9 h 56 m 37". 
Its revolution round the sun, at the mean distance of 
485,000,000 of miles from that luminary, is executed 
in 4332 d .596, in an ellipse of which the plane is 
inclined to that of the ecliptick 1° 46'. The great' dis- 
tance at which this planet is placed prevents our dis- 
tinguishing its phases, which it doubtless exhibits, like 
all the other planets. 

When examined with a telescope, Jupiter is found 
to be attended by four small, luminous bodies, which 
move in regular orbits round it, as our moon does 
round the earth: these are called Jupiter's satellites. 
They are distinguished by their positions, and by num- 
bers, the first being that one which is nearest to the 
planet. They revolve in orbits which are very nearly 
in the plane of the equator, 

The 1st. in l d 18 h 27* 35 s . 

" 2d. " 3 13 13 42. 

" 3d. "73 42 33. 

" 4th. " 16 16 32 8. 

The first three move almost in the same plane, but 
the fourth deviates something more. Their orbits are- 
very nearly circular; no eccentricity has been ob- 
served, except in those of the third and the fourth, and 
of these it is the greatest in the last. 

Herschel, in attentively examining these satellites, 
Avith a telescope, discovered that the intensity of their 
light experienced periodick variations, and in calcula- 
ting the epochs at which their faces are turned towards 
us, he was enabled to determine the duration of their 
revolution upon their axes. He found that they al- 
ways present the same face towards Jupiter; and con- 
sequently they run through their orbits, and revolve 
upon their axes in the same time, as we have already 
seen is the case with our moon. Maraldi had pre- 
viously proved this true of the fourth satellite, by 



JUPITER. 139 

noting the returns of a spot which he had observed 
upon its surface. 

When the satellites of Jupiter, by reason of their 
motion in their orbits, come to be situated between the 
sun and Jupiter, they project, upon the enlightened 
part of that planet's disk a shadow, which varies in 
size, according to the distance and magnitude of the 
satellite producing it. This constitutes a partial eclipse 
of Jupiter; and from the fact here stated we learn, 
with certainty, that neither Jupiter nor its satellites are 
self luminous, but only shine by means of reflecting 
the light of the sun. 

When, on the contrary, the motion of these satel- 
lites carries them to the opposite parts of their orbits, 
and of course behind the planet, relatively to us, we 
see them, then, successively disappear, or in other 
words, become eclipsed by their primary planet. The 
first three are eclipsed at every revolution; but the 
fourth moves in an orbit so much inclined that, in its 
oppositions to Jupiter, it is two years in six without 
falling into its shadow. By the singular relations 
which exist among the motions of the first three of 
these satellites, but which cannot be clearly explained 
here, it follows that these three satellites cannot be 
eclipsed at the same time; for, during the simultane- 
ous eclipse of the second and third, the first is con- 
stantly in conjunction with Jupiter; and reciprocally. 

It has always been observed that these eclipses ne- 
ver take place from east to west, but always upon their 
return from west to east: from which we learn that 
these satellites, like most of the planets of our system, 
move round their primary from west to east. 

These eclipses of the satellites of Jupiter, have fur- 
nished the means, as we shall see hereafter, of deter- 
mining the velocity of light: and we shall also learn 
that they are of great utility to mariners, in determi- 
ning longitude. 



140 OP THE PLANETS. 

PHY«ICAL CONSTITUTION OF JUPITER. 

We have seen that both Jupiter and its satellites bor- 
row their light from the sun. Although this planet 
is 1470 times more voluminous than the earth, its den- 
sity is only one quarter as great as that of our planet. 
Its figure is that of a spheroid, flattened at the poles. 
This ellipticity, which is T ^, is an effect of the rapidity 
of that planet's rotary motion, as we shall demon- 
strate in speaking of the earth. Its axis being nearly 
perpendicular to the plane of its orbit, the sun is al- 
most always in the plane of its equator: consequently 
the variations of the seasons are scarcely perceptible, 
and the days and nights are very nearly of equal length, 
during the entire year. 

The sun, viewed from Jupiter, would appear five 
times less than if seen from the earth; and the heat 
and light which it sends to that planet are five times 
less than we enjoy. But its nights are very short, and 
enlightened by four brilliant moons, of which one, at 
least, is always shining. 

When we examine Jupiter, with a good telescope, 
we perceive a number of zones, or bands, of a darker 
colour than the rest of its disk. They are generally 
parallel to the equator; but in other respects they are 
subject to great variations. Sometimes only a single 
one of these is to be seen, while at others no less than 
eight are visible. Frequently these are not parallel, 
one to another; and often, also, they are of variable 
lengths. Sometimes they are seen to extend, and 
sometimes to contract their dimensions. The time of 
their duration, also, varies much. Some of these bands 
have been seen to endure for three months, without 
change of form or size: while others have been formed, 
or have vanished in the space of one or two hours. At 
times the continuity of these bands is interrupted, giv- 




SATURN. 141 

Fig. 26. ing them the appearance of having 
been broken off, and displaced. The 
spots and the bands which were ob- 
served upon Jupiter's disk on the 7th 
of April, 1792, are represented in 
fig. 25. These bands are supposed 
to be portions of the body of the pla- 
net; and the luminous parts to be 
clouds transported by winds, often 
with great velocity, and in different directions. 



SATURN, £, ITS RINGS AND ITS SATELLITES. 



Viewed with the naked eye Saturn presents to us 
the appearance of a nebulous star, of a dull, leaden 
colour; and as its motion is very slow, it is often, by 
others than astronomers, confounded with the fixed 
stars. It presents, parallel to its equator, a series of 
bands, analogous to those of jupiter, although not as 
dark and distinct; and it was by the aid of these that 
Herschel determined its movement of rotation upon its 
axis, which is accomplished in 10 h 29 m 2 s . It moves, 
at the distance of about 890,000,000 of miles from the 
sun, in an orbit in which it describes its revolution 
round the sun in twenty-nine years, five months and 
fourteen days, and of which the inclination to the eclip- 
tick is 2° 27'. This planet is nearly nine hundred 
times larger than the earth; and the sun dispenses to 
it only about one eighth part of the light which the 
earth receives. 

Like jupiter, Saturn has satellites, but in greater 
number. It has seven — six of which move very nearly 
in the plane of the equator, but the seventh deviates 
very sensibly from this path; the inclination of its or- 
bit being about 30°. Observation has shown that this 
12* 



142 THE PLANETS. 

satellite, like our moon, turns upon its axis in the same 
time that it completes a revolution in its orbit; so 
that it always presents the same part of its surface to 
its primary planet We have not, indeed, been able 
to determine that this is true of the remaining satellites 
of Saturn, but analogy strongly induces to the belief 
that it is: for this equality of duration of the move- 
ments of translation and of revolution appears to be a 
law of the secondary planets. 

The duration of the revolution of the dirTerent satel- 
lites of Saturn is very unequal, as will be seen by the 
following table, in which their mean siderial revolu- 
tions are given, and their mean distances from their 
primary are expressed in the radii of that planet — its 
diameter being 79,000 miles. 



::"S£:.f"^:c. 


Sffril rer.'.c. 


Distance from 


1st 


22 h 37" 




2d. 


l d 8 53 9; 


4.300. 


3d. 


1 Bi 16 




4th. 


3 17 44 51. 




5th. 


4 ie 25 11: 




6th. 


1 14: 


22 



7th. 79 7 54 37; 64 .:. 

The satellites of Saturn experience frequent eclip- 
ses, and these, like those of jupiter, serve for the de- 
tennination of longitude; but their great distance ren- 
ders observations upon them both difficult and uncer- 
tain. 

A law, which is that of Bode's, already noticed, has 
recently been determined to exist among the satellites 
of Saturn. The distances of these from their primary 
may be expressed by the following numbers: 1. 2, 4. 
8, 16, — . 64. There seems to exist a void, according 



SATURN. 



143 



to Bode's law, between the sixth and seventh of these 
satellites, which leaves room to suspect that another 
body, yet undiscovered, revolves there. Such a void 
was long supposed to exist between the orbits, of mars 
and jupiter, which was filled, at the beginning of the 
present century, by the discovery of the telescopick 
planets, which satisfied the law in question. See 
pages 100 and 101. 

Saturn, which is already so remarkable for the num- 
ber of its satellites, is rendered still more so by the 
rings which envelop it; as represented in fig. 26. 

Fig. 26. 




These rings are concentrick; very broad, and thin; 
both lying in the same plane, and that the plane of the 
equator of the planet. To this body they constitute a 
kind of zone, or band, although separated from the 
surface of the planet by a space of many thousands of 
miles, upon every side. Viewed from the earth these 
rings present themselves under an elliptical form, more 
or less elongated, according to the obliquity under 
which they are seen; and which arises from the dif- 
ferent degrees of inclination which the planet Saturn 
assumes, with regard to the earth, while traversing its 
orbit. The following sketches, twelve in number, fig. 
27, illustrate the different appearances of this planet 
and its rings, in the several constellations. 



144 



OF THE PLANETS. 

Fig. 27. 




CO 



v d 




SI 



n 




V3 



X 



By these figures it will be seen that the rings are 
never so situated with regard to the earth as that the 
plane of them should lie at right angles to our line of 
vision; but that we always view their sides under a 
greater or less degree of obliquity. Were they placed 
with the side fully presented to our sight, they would 
then be deprived of that elliptical appearance which 
now distinguishes them, and assume a circular form. 
There are several of the positions shown in the figures, 
as that in n» a *id in So? where stars may be seen be- 
tween the planet and the inner ring; while in C P> — , 
&c. the obliquity is so great as to prevent this, As 



SATURN. 145 

some of these figures exhibit one side of the rings to 
view, and some the other, it follows that there must be 
times when neither side is visible. This occurs when 
their position is such as that the prolongation of their 
plane passes through the centre of the earth. At such 
a time the edge of the outer ring, only, is presented to 
us; and this is so thin, (about one hundred miles,) and 
the angle which it subtends is so small that telescopes 
of very great light and high magnifying power are 
necessary to render it visible. When thus seen, it 
resembles a luminous line or thread, crossing the disk 
of the planet. This phenomenon occurs once in every 
fifteen years. 

By employing very powerful telescopes we discover, 
upon the surface of Saturn's ring, several concentrick, 
black lines, appearing like divisions; and one of these 
was clearly determined to be such, by Herschel, who 
calculated its dimensions, and thus established the ex- 
istence of two distinct rings. According to this astro- 
nomer the following dimensions exist: 

Interval between the planet and the 

inner ring, 19,090 miles. 

Inner diameter of the inner ring, 117,339 " 

Outer diameter of the same, 151,690 " 

Inner diameter of the outer ring, 155,272 " 

Outer diameter of the same, 176,418 " 

Interval between the two rings, 1,791 " 

Thickness of the rings, 100 " 

By means of spots of unequal brightness, upon the 
rings, Herschel determined the duration of their revo- 
lution upon their axis to be 10 h 29 m 16 s ; which differs 
but little, and probably no more than the unavoidable 
errour of observation, from the time assigned them by 
Laplace, as deduced from the laws of gravity. Their 
axis of rotation is perpendicular to their plane, and is 
the same as that of Saturn. 



146 OF THE PL a: 

The duration of this rotation, which appears to be 
precisely that of a satellite which should have for its 
orbit the mean circumference of the ring, has furnished 
Mons. Biot with cava where? :plain how the 

ring's of Saturn are sustained in their position around 
that planer, without touching it; or rather, with the 
means 01 connecting this fact with the general cause 
which equally sustains all satelli: : 

This he does by showing that each particle of which 
the rings are composed m usidered a small but 

distinct satellite of Saturn; and t: ; lives. 

therefore, are no other than a united 

together in an unchangeable manner. If these parti- 
cles were all detached from each other, their velocity- 
would vary with their distance from the centre of the 
primary planet, those nearest to this point movin g 
the greatest, and those most remote, with the le: 
locity : and if we take, for the mean term, the velocity 
which pertains to the mean circumferen 
the velocity of the other pa: old differ from 

this (some more and son: y an equal qu: 

Now, if all these particles be united, and attached, so 
as to form a solid body, a kind of compel 
their several movements would be the result; the most 
rapid would communicate to those moving more slowly 
a portion of their velocity, at the same time that the 
slower would assist in retarding these; the opposite 
forces thus tending to produce an equilibrium, until the 
whole would result in a mean n>: :ommon to 

all the particles, which would be that of the mean cir- 
cumference. These rings ed in their 
place, around Saturn, just as the moon is sustained in 
its place round the earth. 

This theory still holds good, if we suppose Saturn's 
appendage to be composed, as we shall soon i 
really is, of several concentrick rings, separate and 
detached from each other: only, in that case, it would 



SATURN. 147 

be necessary to apply it separately to each of them, 
because the duration of their rotation would be sensi- 
bly different. 

Sometimes the rings of Saturn are projected upon 
the disk of the planet, so as to hide a portion of it from 
our sight, and at others the planet so intervenes as in 
like manner to eclipse a portion of the rings. Exam- 
ples of both these results will be seen in the diagrams, 
rig. 27, page 144. From this it follows that both the 
planet and its rings are solid, opaque bodies, borrow- 
ing their light from the sun. 

The rings, though very nearly concentrick with the 
planet, are not strictly so; their centre of gravity be- 
ing found to describe, around the centre of gravity of 
the planet, a very minute orbit. This fact, it has been 
demonstrated, is indispensable to the stability of the 
rings; and but for that, they would be thrown down 
upon the planet, so that, upon some side their inner 
edge would come in contact with its surface, where it 
would then inevitably remain. It is worthy of remark, 
also, that in the progress of Saturn in its orbit, as the 
rings go with it, each of these rings and the planet 
itself must needs have exactly the same projectile ve- 
locity, since the slightest difference would speedily de- 
range and destroy the whole adjustment. 

The foregoing embraces all that astronomers knew 
of Saturn and its appendages, prior to the year 1838. 
Several astronomers had previously suspected the rings 
of Saturn to be more than two in number; but with the 
telescopes they employed it was impossible to decide. 
Laplace, as early as 1787, supposed the' ring of Sa- 
turn might have many divisions; while Herschel con- 
tended that no observations would justify this supposi- 
tion. But a new telescope, more powerful and perfect 
than any previous one, now in use at Rome, in Italy, 
appears to have set this point at rest. This telescope 
was made by Cauchoix; and with it, on the night of 



148 OP THE PLANETS. 

the 29th of May, 1838, the astronomers of the Roman 
College, at Rome, first suspected they had new proof of 
additional divisions in the rings of Saturn. Their ob- 
servations were continued; and the evidence becoming 
stronger, they invited jlons. Decuppis to assist in pur- 
suing the subject. From the published result of these 
investigations there appears no doubt that the rings of 
Saturn are six in number, instead of two, as Herschel 
supposed them. The figure in front of the titlepage 
has been prepared from the description given of these 
discoveries; and it is drawn with the plane of the rings 
at right angles to the line of vision, because the differ- 
ent proportions are much more perfectly shown, and 
will be better understood in this way; although the 
earth never occupies a position which commands such 
a view of Saturn and its rings, in the heavens. In 
this diagram the white portions, or bands, surrounding 
the planet, represent Saturn's rings, while the dark 
ones, some of which appear only as lines, denote the 
spaces between the rings, and between the inner ring 
and the body of the planet. 

The following are the results of these discoveries, 
as published by the French Academy of Sciences, in a 
tabular form. 

TABLE OF THE APPROXIMATE DIMENSIONS OF SATURN 
AND ITS RINGS. 

Miles. 
Equatorial diameter of the planet, 79,000 

Interval between Saturn and the interiour ring, 18,478 
Internal diameter of the interiour ring, 117,311 

Diameter of the first division, 125,539 

Diameter of the second division, 137,412 

Diameter of the third division, 145,611? 

External diameter of the interiour ring, 153,547 

Interval between the two rings, 1,914? 

Internal diameter of the exteriour ring, 155,233 

Diameter of the fourth division, 166,453 

External diameter of the exteriour ring, 176,376 



URANUS. 



149 



In this table the two rings, as previously known, 
and the interval between them, are constantly spoken 
of, while the new partitions are laid down as divisions 
of those rings. The space between the planet and the 
inner ring, and also that between the two rings, as 
previously known, are laid down in their due propor- 
tions; but the rest of the divisions between the several 
rings, as we have no means of ascertaining their width, 
are shown only by lines. 

In 1805 Sir William Herschel believed he had de- 
tected a peculiarity in the figure of Saturn, which he 
subsequently proved, by his method of calculations. 
The result of his observations and calculations was, 
that the greatest diameter of this planet is not at the 
equator, as in the other planets, but through the lati- 
tude 43° 20': and this deviation from the form of the 
other planets he supposed to arise from the attraction 
of the ring. The results thus obtained Herschel pub- 
lished, in the Philosophical Transactions of the Royal 

Fig. 28. 




Society, in 1806, where he illustrated his theory by a 
drawing of the planet; and which we have copied here, 
figure 28. 

13 



150 OF THE PLANETS. 

The name of Herschel is of so great weight, in as- 
tronomy, that this supposed discovery of his has been 
promptly adopted, in this country-, to some extent, at 
least; and has found its way into text books, of high 
standing, whose authors unfortunately had not met 
with its entire refutation. Such refutation, however, 
was furnished in 1S32, by Mons. Be<sel, in a determi- 
nation of the dimensions and position of the ri: . 
Saturn, and the dimensions and form of that planet 
With particular reference to Herschel's publication 
upon this subject, and with instruments far more per- 
fect than those possessed by Herschel, Bessel carefully 
measured the diameter of this planet, at the equator, 
and at latitudes 22 : 30 . 45 : . 67- 30 . and 90 2 . From 
these measures it is established that the greatest dia- 
meter of Saturn is through its equator, and its least 
is through its poles — as is the case with all the 
other planets, so far as known. The agency of the 
rings of Saturn, too, in producing the shape Herschel 
supposed that planet to have, is :■: laBy lis Droved by 
theory, as by admeasurement. The result, then, both 
of direct measurement, and of calculations, says Mons. 
Bessel, is in positive contradiction to the figure c 
turn, given by Herschel. 

URANUS, T§.. 

This planet is, of all those belonging to the solar 
system, the most elongated from the sun, and its orbit 
therefore envelops those of all the others. Situated at 
1.500.000.000 miles from the sun it accomplishes its 
revolution in its orbit in 54 years, 29 days. 3 hours 
and 39 minutes. The inclination of its orbit to the 
plane of the ecliptick is less than that of any other 
planet, being only 46 26 . The period of the diurnal 
rotation of this planet has not been determined. 

To the naked eye Uranus is barely visible: in the 
telescope its disk is well denned, and appears of a blu- 



URANUS. 151 

ish white colour. Owing to the great distance at which 
this planet is placed from the centre of the solar sys- 
tem, it receives only the three hundred and sixty-sec- 
ond part of the light from the sun which that body 
emits to the planet we inhabit. 

Although often seen, and several times entered in 
catalogues of fixed stars, as one of their number, yet 
Uranus was never supposed to be a planet, or to be- 
long to our solar system, until the year 1781. As no 
planet was suspected to exist beyond saturn, so none 
was sought for there; nor had the discovery of motion, 
in Uranus, the most distant connexion with any pre- 
conceived intention of such discovery: it was the re- 
sult of accident alone. On the night of the 13th of 
March, in the year above mentioned, Herschel, who 
was a native of Hanover, and who resided in England, 
while engaged in a series of observations upon the par- 
allax of the fixed stars, and regarding with attention 
several near the feet of Gemini, he was struck with 
the fact that one appeared larger than the rest, when 
seen through his telescope. As the fixed stars are not 
magnified by these instruments, the fact of its enlarged 
appearance, when once detected, fixed the observer's 
attention. The telescope with which this discovery 
was made was one of seven feet in length, of the re- 
flecting form; and the eye piece in use at the moment 
was one which magnified only two hundred and twenty- 
seven times. Having, by subsequent observations, 
determined that the body in question had changed place, 
in relation to those by which it was surrounded, Her- 
schel no longer concealed his discovery. But at this 
time he had no suspicion that he had discovered a 
planet. He wrote to the Royal Society, of which he 
was a member, stating the facts, and adding that his 
first impression was he had detected a small comet, 
without either tail or envelope; and in a subsequent 
part of his announcement he adds: "the sequel has 



152 OF THE PLANETS. 

shown that my surmises were well founded, this prov- 
ing to be the comet we have lately observed." 

This announcement was communicated, by Maske- 
lyne, the English astronomer, to Mons. Messier, and 
then the astronomers of Paris were at once engaged in 
observing the supposed comet, and in calculating its 
orbit: for Herschel furnished no calculations, whatever, 
of the orbit of his supposed comet; nor were his observa- 
tions so made that any aid could be derived from them, 
in such calculations. Nor were they long in detecting 
the errour which Herschel had committed, in regard to 
the body in question. On the 8th of May, 1781, less 
than two months from the first discovery of Uranus, as a 
moving body, Jean-Baptiste Gasper Bochart de Saron 
ascertained that it was much more elongated from the 
sun than any of the other planets; and his extraordi- 
nary facility, in calculating cometary orbits, had thus 
early enabled him to know that the motions of this 
body did not conform to a parabolick curve. He then 
gave the first idea of a circular orbit; and this sug- 
gestion was carried out, and the orbit itself determined, 
by his co-labourer, Mechain, according to the method 
of Laplace. 

Thus, through the combined agency of a most 
happy piece of unexpected good fortune, on the part 
of Herschel, in England, and the industry and mathe- 
matical skill of the Paris astronomers, a new planet 
stood revealed to the knowledge of mankind, belonging 
to our solar family, and yet revolving in an orbit so 
immensely distant as to envelop all the others, and 
to give to the known limits of the solar system an 
augmentation of dimensions almost beyond conception. 

According to Herschel there are six satellites be- 
longing to Uranus, which achieve their revolutions in 
their several orbits, round their primary, in the fol- 
lowing periods of time, respectively: 



URANUS. 153 

The first achieves its siderial revolution in the 
space of 

5 d 21 h 25-n 21 s . 

The 2d. 8 16 57 47. 

" 3d. 10 23 3 59. 

" 4th. 13 10 56 30. 

" 5th. 38 1 48 00. 

" 6th. 107 16 39 56. 

These bodies are so small and so immensely dis- 
tant, that there necessarily exists much uncertainty 
respecting them. The greatest certainty, by far, 
exists with regard to the 2d and 4th of these satellites, 
and they present some most extraordinary features, 
nowhere else met with among the bodies of our solar 
system. The planes of their orbits are almost per- 
pendicular to the ecliptick, being inclined no less than 
78° 58'; and in their orbits they move from east to 
west, or in the direction contrary to every other planet 
that is known to us, whether primary or secondary!* 

It has often been averred — and it is necessary here 
to repeat the correction, because the errours in question 
are still found in the newest books — that Dr. Herschel 
discovered motion in Uranus through the agency of 
his noted forty feet telescope; and also that he recog- 
nised that body as a planet. For the promulgation of 
these errours, numerous works, of commanding in- 
fluence, are responsible, not to enumerate a multitude 
of minor publications, some of them school books, and 
hence in the hands of many of our youth, which have 
given them currency until they have well nigh passed 
into proverbs. Dr. Herschel, himself, in his publica- 
tions, upon these subjects, has given no authority for 
these fabulous creations of some unknown pen. 

* Among those fixed star3 that are known to have motions, in 
regular orbits, there is no such uniformity of direction; some of 
them being found to have a direct, and others a retrograde motion. 
13* 



154 OP THE PLANETS. 

Herschel once supposed that he had discovered two 
rings, surrounding Uranus, like those which envelop 
saturn; but he subsequently abandoned all belief in 
their existence. They were evidently optical illusions; 
and it is highly probable that they arose from the de- 
fect of figure of the mirror of the forty feet telescope. 
Indeed this instrument, which has been so often and so 
constantly the theme of eulogy and admiration, seems 
never to have been of very extensive practical use, 
particularly where the forms of heavenly bodies were 
to be determined. The figure of the mirror was so 
defective that the images of the celestial bodies, which 
it produced, were greatly distorted; and for this rea- 
son the magnifying power used upon it seldom ex- 
ceeded two hundred — high powers, of course, greatly 
augmenting the distortion. This telescope was taken 
down, some years since, and laid aside; for which no 
other, or more sufficient reason has been assigned, than 
that the frame work, which supported it, had become 
decayed! No farther use has been made of this in- 
strument; a much smaller one having been erected in 
its place, at the time of its removal, which still occu- 
pies that position. 

The following tables exhibit, at a single view, a va- 
riety of useful and necessary details, with regard to 
both the primary and secondary planets. 

DISTANCES OF THE PLANETS FROM THE SUN. 

Names of Distances, in miles, | Names of Distances, in miles, 

the planets. in round numbers. | the planets. in round numbers. 

Mercury, 37,000,000 Juno, 1 

Venus, 68,000,000 Ceres, C m ^ n 261,000,000 

The Earth, 95,000,000 Pallas, ) 
Mars, 142,000,000 Jupiter, 485,000,000 

Vesta, • 225,000,000 Saturn, 890,000,000 

Uranus, 1,800,000,000 



TABLES. 



155 



For more accurate results, the following table of 
the same may be referred to. 



MEAN DISTANCES 


FROM THE SUA 


, THAT TO THE EARTH BEING 1. 


Mercury, 


.3870981 Ceres, 2 .7672450 


Venus, 


.7233316 


Pallas, 2 .7728860 


Earth, 


1 .0000000 


Jupiter, 5 .2027760 


Mars, 


1 .5236923 


Saturn, 9 .5387861 


Vesta, 


2 .3678700 


Uranus, 19 .1823900 


Juno, 


2 .6690090 





VOLUMES O* 


• THE 


SUN AND PLANETS, 
EARTH BEING 1. 


THAT OF THE 


The Sun, 


1,328,460 .00 


Vesta, ~) 




Mercury, 




0.10 


Juno, I TT , 


Venus, 




.90 


Ceres, ( 




The Earth, 




1 .00 


Pallas, 3 




The Moon, 




0.02 


Jupiter, 


1,470 .20 


Mars, 




.20 


Saturn, 
Uranus, 


887 .30 
77 .50 



Unknown. 



MASSES OP THE SUN AND OF THE PLANETS, THAT OP 
THE EARTH BEING 1. 

The Sun, 337,086 .0000 Vesta, 1 
Mercury, .1664 Juno, ! 

Venus, .9452 Ceres, f 

The Earth, 1 .0000 Pallas, J 

The Moon, .0170 Jupiter, 315 .8926 

Mars, .1324 Saturn, 120.0782 

Uranus, 17 .2829 



DENSITIES OF THE SUN AND THE PLANETS, THAT OF 
THE EARTH BEING 1. 



The Sun, 
Mercury? 
Venus, 
The Earth, 
The Moon, 
Mars, 



.236240 
2 .879646 

1 .047010 
1 .000000 
.715076 
.930736 



Vesta, 

Juno, 

Ceres, 

Pallas. 

Jupiter, 

Saturn, 

Uranus, 



.1 



Unknown. 



.241190 
.095684 
n .020802 



I5C 






f FZET. PER SE -TLXQ 

BODY WOULD PASS THSOrG 7 THE 






p.: Zirrh. 
The Moon. 

Ves;~. Vr.I-:r_:"T.. 



.:: 

12 
18 

16 
3 



own. 

jJopiter, 

iSaturn, 



-r- 



TEMES OF BOTAT 


r?o> 






r::i 


iiaxzt : 






The Sun, : T 








Mercurv, 1 
5 


5 


fano, j T - - 


Tne Earth, 1 











The Moon, 21 7 


__ 





j Jupiter, 


" 


Mars, 1 


39 






10 


29 2 








Uranus, Unknown. 


t:::is :r tei 


7.ZV 


: ltt: 








:o 













87 


14= 


30- 


Vrz.-J = . 







16 


41 




The Earth, 







5 


48 


49 


" . - -. ■ 










42 


Vesta, 




3 


66 4 








Juno, 




4 








8 


Crl^E. 




4 


•; 


D 









4 


16 








.".:-::?;. 




11 


12 


30 











161 







NB, 




B4 


:. 


39 






" :- QCS, 

Tr.r :.:-.:■-. 

Mars, 



Iter, 

:: 

1 ! 






TABLES. 



157 



INCLINATION OP THE RESPECTIVE PLANETARY ORBITS 
TO THE ECLIPTICK. 



Mercury, 


7° 


78' 


Ceres, 


10° 


37' 


Venus, 


8 


76 


Pallas, 


34 


37 


The Moon, 


5 


71 


Jupiter, 


1 


46 


Mars, 


1 


51 


Saturn, 


2 


77 


Vesta, 


7 


8 


Uranus, 





46 


Juno, 


13 


5 









INCLINATION OF THE AXES OF THE SUN AND PLANETS 
TO THEIR ORBITS. 



The Sun, 


82° 50' 


Vesta, ' 7 


Mercury, 
Venus, 


— — 


n ' > Unknown. 
Ceres, \ 


The Earth, 


66 52 


Pallas, j 


The Moon, 


88 50 


Jupiter, 89° 45' 


Mars, 


61 33 


Saturn, 60 
Uranus, — — 



LEAGUES, IN ROUND NUMBERS, PASSED OVER BY EACH 



Mercury, 635 


Juno, 


— 


Venus, 485 


Ceres, 


— 


The Earth, 412 


Pallas, 





The M00n,rdalively to the earih, 14 


Jupiter, 


178 


Mars, 329 


Saturn, 


132 


Vesta, — 


Uranus, 


93 



SATELLITES OF JUPITER. 



Mean distances, the demi- 


Duration 


Mases of the satel- 


diameter of the planet 


of the 


lites, that of the 


beinir 1. 


revolutions. 


planet being 1. 


1st. {Satellite. 6 .0485 


Id .7691 


.000017 


2d. do. 9 .6235 


3 .5512 


.000023 


3d. do. 15 .3502 


7 .1546 


.000088 


4th. do. 26 .9983 


16 .6888 


.000043 



158 



LAWS OF KEPLER. 



SATELLITES OF SATURN. 



Mean distances, the demi-diameter of the 
planet being 1. 



Duration of the 
. revolutions. 



1st Satellite, 


3 .35 


Qd .943 


2d. do. 


4.30 


1 .370 


3d. do. 


5 .28 


1 .888 


4th. do. 


6 .82 


2 .739 


5th. do. 


9.52 


4 .517 


6th. do. 


22 .08 


15 .945 


7th. do. 


64.36 


79 .330 



SATELLITES OF URANUS. 



Mean distances, 


the derni- diameter of the 


Duration of the 




planet being 1. 




revolutions. 


1st. Satellite. 






13 .12 


5d .S93 


2d. do. 






17 .02 


8 .707 


3d. do. 






19 .85 


10 .961 


4th. do. 






22 .75 


13 .456 


5th. do. 






45.51 


38 .075 


6th. do. 






91 .01 


107 .694 



CHAPTER EIGHTH. 



LAWS OF KEPLER. 



We have heretofore contented ourselves, while treat- 
ing of the planets, with stating that they describe, 
round the sun eliptical curves, more or less elongated; 
but we have not yet examined into the means of deter- 
mining these orbits; nor have we investigated their na 
ture. 

The curves described by the planets always make 
an angle, more or less open, with the plane of the 
ecliptick, and they consequently cut this plane, at two 
points exactly opposite. These points are the nodes; 



LAWS OF KEPLER. 159 

and a line joining these two points is the line of the 
nodes. This line determines the trace of the plane of 
the planet's orbit upon the ecliptick. Let us now sup- 
pose an observer to be placed upon the sun: it would 
be easy for him to know the precise instants of the 
passage of the planet through its nodes; which would 
take place when the planet should be upon a line pass- 
ing through the node and the centre of the sun. As 
for an observer upon the earth; that is, out of the cen- 
tre of the solar system, he can very well fix the mo- 
ments of the passages of the planets through their nodes, 
although he cannot observe them, as from the sun, be- 
cause the right line which joins them must successive- 
ly assume various inclinations, by the effect of the 
sun's motion; yet it sometimes, though very rarely, 
happens that, the sun and the earth being upon the 
same line, the planet to be observed will be found upon 
the prolongation of that line. It will then be seen 
at the same point as the sun; the observer may fix its 
longitude, and several observations of this kind will 
determine if the node of the planet always corresponds 
to the same longitude, seen from the sun. 

The node being found, to determine the inclination 
we seize the moment when the sun has the same lon- 
gitude as the planet; we then obtain the latitude of the 
body, from which we deduce the inclination of the 
plane of the orbit. 

After these data are obtained, in order to find the 
nature of the curve, we measure the duration of an 
entire revolution; which is done by fixing upon some 
point, one of the nodes, for example, and carefully 
computing the time which elapses between two suc- 
cessive passages of the planet through this point 

When w T e have thus obtained the duration of the 
revolution nothing remains but to fix, by means of the 
oppositions and conjunctions, the angular movement 
of the planet. 



160 UNIVERSAL ATTRACTION. 

Having thus traced the orbits of the planets we 
learn, 

1st. That all the planets move in ellipses, of which 
the sun occupies one of the foci; 

2d. That the motion is more rapid in proportion as 
the planet is nearer the sun, so that the radius vector, 
or imaginary line joining the centres of the sun and 
planet, always describes equal areas in equal times; 

3d. That the squares of the times of the revolutions 
are as the cubes of the grand axes of the orbits. 

These are the laws of Kepler; and they serve as the 
basis of astronomy. We shall see, hereafter, how 
these laws contain the germ of the general law of 
attraction. These important laws have been verified 
in the case of all the planets; and they are found so 
perfectly exact that we no longer hesitate to fix the 
distances of the planets from the sun by the duration 
of their siderial revolutions: and we know that this 
method of determining distances affords us great exac- 
titude, for it is always easy to determine, with preci- 
sion, the return of each planet to a given point in the 
heavens, while it is very difficult to calculate, directly, 
its distance from the sun. 

UNIVERSAL ATTRACTION. 

The laws of Kepler, which have rendered so impor- 
tant services to astronomy, in disclosing the wonder- 
ful relations of the celestial movements, were particu- 
larly calculated to excite inquiring minds to an inves- 
tigation of the cause which presides over these move- 
ments. This discovery was reserved for the genius of 
Newton; but the details and the calculations by which 
he established the existence of this general cause would 
be useless to the pupil here; and we shall therefore 
limit ourselves to an exposition of the consequences 
which he deduced from the laws of Kepler. 



I MVERSAL ATTRACTION. 161 

1st. From the fact that the radius vector always 
describes equal areas in equal times, Newton deduced, 
and supported by calculations, the consequence that 
the force which holds the planets in their orbits acts in 
the direction of the centre of the sun. 

•2d. Kepler had shown that the orbits of the planets 
are ellipses, of which the sun occupies one of the foci; 
and Newton, from this, first inferred, and afterwards 
proved, that the force which acts upon the planets is in 
the inverse ratio of the square of the distance of their 
several centres from that of the sun. And, 

3d. From the fact that the squares of the times of 
the revolutions are as the cubes of the transverse 
axis of the orbits, he deduced this consequence, namely, 
that the force is proportional to the mass. 

From these several results it follows that the sun is 
the centre of an attractive power which acts in virtue 
of the foregoing laws. 

Newton, who, after noting the attraction exercised 
by the earth, upon bodies at its surface, extended the 
agency of this attraction to the moon, subsequently 
concluded, from analogy, since the other planets re- 
tained their satellites in their several orbits, as the 
earth holds the moon in hers, that those planets must 
possess, like the earth, an attractive force; and that 
this force could be no other than one of the same na- 
ture as that which gives to the sun the power to cause 
to revolve around him, in well defined orbits, all the 
bodies belonging to the solar system. 

The several bodies which revolve around the sun 
are, like him, endowed with the power of attraction, 
and by carrying the analogy farther we arrive at this 
general result, which physicks has fully demonstrated, 
and of which the special form of the celestial bodies 
gives strong presumptive evidence, namely, that all 
particles of matter mutually attract each other, in the 
14 



162 



UNIVERSAL ATTRACTION. 



direct ratio of their masses; and reciprocally, as the 
squares of their distances. 

But, as the force of attraction, if it existed alone, 
would directly tend to assemble, in one mass, and there 
to retain, all the globes which the universe embraces, 
Newton supposed that the celestial bodies had primi- 
tively received an impulsion, in a right line, and that 
from the combination of these two forces results the 
curvilineal motion of the planetary bodies. 

To illustrate this; If the body A, fig. 29, is projected 
upon the right line A B X, in open space, where it 
Fig. 29. 




shall encounter no resistance to enfeeble the impulsion 
which has been given to it, that body, under these cir- 
cumstances, would continue, indefinitely, to move with 
the same velocity, and in the game direction. But if, 
on arriving at B, it is attracted by S with an adequate 
force, acting perpendicularly to its line of motion, it 
will be drawn from the right line A B X, and will de- 



UNIVERSAL ATTRACTION. 163 

scribe around S the circle B Y T U. That the body 
may describe this circle, it is necessary that the pro- 
jectile force with which it be impelled upon the right 
line A B X should be equal to that which it would 
have acquired, by gravity alone, in falling through 
half the radius of that circle. 

That this body, -when arrived at B, should describe 
the circle B Y T U, it is necessary that it should be 
so attracted by S as to fall from B to y, half the dis- 
tance B X, in the time it would take to move from B to 
X by the effect of the force of projection alone. In this 
case, we may suppose A a planet and S the sun. 

But if, while the projectile force carries the planet 
from B to b, the attraction of the sun cause it to fall 
from B to 1, the power of gravitation would be propor- 
tionably greater than in the previous case; and the 
planet would then describe the curve B C. When the 
planet, in this curve, shall have arrived at C. gravita- 
tion, which augments in the inverse ratio of the squares 
of the distances, will act more powerfully than at B, 
and thus cause the planet to descend with greater velo- 
city, so as to make it describe the arcs B C, C D, D E, 
E F, F I, I K, in equal times: the planet moving, in 
this case, with much greater rapidity than before, it 
would necessarily acquire a great tendency to fly off 
in the tangent K k, or, in other words, its projectile 
force would, in this way, be increased to such an ex- 
tent as would be sufficient to overcome the force of 
attraction, and thus prevent the planet's falling into the 
sun; or even from moving in the circle K/wn. The 
planet, then, would increase its distance from the sun, 
in following the curve K L B, but its velocity would 
gradually decrease from K to B, as it had previously 
increased from B to K, because the solar attraction 
would here be exercised in the opposite direction. 
Once more at B, after having lost, in moving from K 
to B the excess of velocity which it had acquired in 



164 UNIVERSAL ATTRACTION. 

passing from B to K, the planet would still obey the 
same forces, and therefore again describe the same 
curve round the sun. 

A double 'projectile force balances a quadruple attrac- 
tive one. Suppose, then, that the planet, at B, had, 
towards X, an impulsion twice as great as we have as- 
sumed; that is to say, an impulsion which would cause 
it to pass from B to c in the same time we have suppo- 
sed it to consume in passing from B to b. In this 
case it would be necessary that the force of gravity 
should be four times greater in order to retain the pla- 
net in its orbit; in other words, such force would re- 
quire to be capable of causing the planet to fall from 
B to 4 in the same time that the projectile force would 
have carried it, in a right line, from B to c; otherwise 
the planet could not describe the curve B D, as shown 
in the figure. 

As the planets alternately approach the sun, and 
recede from it, at each revolution, there is a little diffi- 
culty in conceiving how, in the former case, they 
should not continue this approximation, when once be- 
gun, until they fall upon the sun's surface; and how, 
in the second, they should not continue for ever to 
augment their distance from the sun; but this difficulty 
vanishes from the moment that we examine into the 
action of the moving forces, and their respective inten- 
sities, in the case in question. The planet, we have 
said, moved by a projectile force which would carry it 
from B to b in the time in which the sun would cause 
it to fall from B to 1, submits to the mutual action of 
these two forces and describes the curve B C. But 
when the planet shall have arrived at K, what will 
then be the action of these forces'? K S being equal to 
the half of B S, the planet will be twice as near the 
sun; the action of gravity will then be four times greater, 
according to the principle already laid down. Conse- 
quently, it will cause the planet to fall from K to V, 



UNIVERSAL ATTRACTION. 165 

in the same time that, at B it would have drawn it 
from B to 1; K V being four times greater than B 1. 
But the projectile force, at this moment, is such as to 
carry the planet, in the same time, from K to k, a 
space double that of B b, as shown in the figure; and 
consequently this force is here double what it was at 
B. Now, we have already stated that a double pro- 
jectile force balances a quadruple attractive force; con- 
sequently the equilibrium, between the two forces, 
would not here be interrupted, and therefore the pla- 
net would continue its route from K to L, &c. in con- 
formity with the resultant of these two forces. When 
the planet shall have reached the point B, it will then 
be again subject to the same forces which first caused 
it to describe the orbit just traversed; and as these for- 
ces continue to act with the same intensity as before, 
the planet will continue to describe, indefinitely, the 
same curve around S. 

Such is the great principle of Universal Attraction. 
It is so exact that there are no perturbations, no devi- 
ations, however slight they may be, for which it does 
not account with the most rigorous precision. Astro- 
nomers have such entire confidence in the results of 
this principle that, when observations are found not to 
agree with the results of calculation, they invariably 
suspect such observations to partake of inaccuracy, 
rather than doubt the settled laws of attraction; and 
thus far, all results show that these suspicions have 
been just. 

OF THE MASSES OP THE PLANETS. 

It is through the aid of the principle of attraction 
that we arrive at a knowledge of the mass and density 
of the sun, and of the several planets — both of which 
we have given, in their place — with much of the other 
knowledge we have of the several globes of our solar 
14* 



— 



166 MASSES OF THE PLANETS. 

system. In as much as the velocity of revolution of 
the satellites depends upon the attractive power oi the 
primary planet, we are enabled to deduce their masses 
from this velocity. If a planet has no satellite, its 
mass is determined by the perturbations which other 
bodies produce in its movements. 

The mass and the volume once ascertained, it is easy 
to obtain the density; as, for this result, it is only ne- 
cessary to divide the mass by the volume. 

Cavendish has determined the mass of the earth by 
another method, which, however, is no less dependant 
upon the principle of gravity. He suspended, from 
the extremity of a very fine thread, a needle which, in 
this situation, was in a condition to yield to the slight- 
est attraction. Near this needle he placed a globe of 
lead, which of course attracted the needle. Under 
these circumstances he carefully determined the dura- 
tion of the oscillations which he caused the needle to 
perform. These he subsequently compared with the 
oscillations of a pendulum, when subject to terrestrial 
gravity, alone, and from this comparison he deduced 
the relative proportion of the attractive force of the 
sphere of lead — the mass of which was known — to that 
of the earth, and thus he found the proportion which 
the mass of the leaden sphere bore to the mass of our 
gbbe. 

Finally, we shall learn, while treating of the earth, 
that gravity has furnished the means of determining 
measures with a precision that is not attainable by any 
other method. 



THE EARTH. 167 

CHAPTER NINTH. 

THE EARTH, ©. 

We omitted to treat of the Earth, in detail, in its 
place among the planets, because a knowledge of 
the foregoing chapters was necessary, to the pupil; 
before entering upon this subject. We will now exa- 
mine, successively, the figure, the dimensions and the 
movements of the earth. 

FIGURE OP THE EARTH. 

Deceived by the illusion of the senses man long re- 
garded the earth as a boundless plain. Subsequently 
this errour was slowly dissipated, by the observation 
of numerous facts. It was noticed, in the level coun- 
tries of the east, in approaching very elevated objects, 
which were at great distances, that the summits of such 
objects were first visible, and that, as the distance to 
the objects lessened, more and more of the parts below 
became revealed to sight, while the base was always 
the part last discovered. These phenomena could not 
be the accidental result of the shape of the face of the 
country, or of any particular circumstance, for it was 
remarked alike in all directions, and in every place, 
and it was always the more sensible in proportion as 
the atmosphere was more pure. Subsequently the 
same observations were made upon the sea; and here 
the phenomenon was still more conclusive, since there 
were neither inequalities nor obstructions, of any kind, 
to favour any supposed deception: and yet it was evi- 
dent enough that the surface of the ocean must neces- 
sarily conform to the general figure of the globe. At 
this day almost every one has either seen or heard 
that a vessel, on leaving shore appears to sink beneath 
the surface of the water, so that her lower parts disap- 



168 THE EARTH. 

pear first, and afterwards, successively, those which 
are more elevated: and last of all the tops of her masts. 
So persons on shipboard, when approaching port, first 
discover the tops of objects, on shore, but do not see 
their inferiour portions, or the surface of the ground 
upon which they stand, until much nearer to them. 
This shape of the earth is farther shown, also, by the 
facts that the tops of mountains are the parts first en- 
lightened by the sun, in the morning, and are still illu- 
mined by the rays of that body, at night, after the sun 
is below the horizon. More recently, still, the con- 
vexity of the earth's surface has been demonstrated, by 
various methods: 

1st By the voyages of navig ho, after having 

made the tour of the earth, have returned to their 
point of original departure, by an opposite direction 
to that in which they departed; 

2d. By astronomical observations: among others, 
that of the circular form of the shadow projected by 
the earth upon the disk of the moon, during an eclipse 
of this last body: and, 

3d. By operations Avhich have served to determine 
the dimensions of the earth: as. for instance, the di- 
rection of the plumb line, at different stations upon 
the earth's surface. 

From proofs incontrovertible, then, we know that 
the earth is very nearly spherical: we say very m 
because we shall soon see that it has the figure of a 
sphere, but flattened at the poles, and enlarged at the 
equator. We shall arrive at these facts while deter- 
mining the dimensions of the globe: and we shall sub- 
sequently see that this form is a necessary effect of 
the earth's movement of rotation. 

DI3IEXSIOXS OF THE EARTH. 

The earth having sensibly the form of a sphere, it 
follows that if we know the length of a single one of 



THE EARTH. 169 

its degrees, by multiplying this by 360, we shall ob- 
tain the circumference, and hence the diameter, the 
surface, and the volume of the earth. This result, 
then, depends upon the determination of a single ter- 
restrial degree. Now, to determine this, in a practi- 
cal manner, the following method has been pursued: 
a space has been selected upon the earth, such, that 
perpendiculars, determined by a plumb line, at each 
of the extremities of this space will correspond to two 
stars situated one degree apart; and the space thus 
designated of course constitutes a terrestrial degree. 
Now, it is easy to see that where this exact distance 
is not readily to be had, no difficulty can arise from 
taking a greater or less space, as simple proportion 
will always give the exact length of a degree. It only 
remains, then, to measure, with precision, the base 
thus chosen. This has been done, with incredible 
exactness, by what is known as the trigonometrical 
method, but which we cannot illustrate here. 

This practical determination of terrestrial degrees 
has confirmed the flattening of the earth, at the poles, 
and its enlargement, or swelling, at the equator. In- 
deed the degree, or the space constituting a degree, 
is not the same, in all latitudes: it is longer in pro- 
portion as we approach either of the poles, while it is 
at its maximum, at the equator: which clearly indi- 
cates a flattening, at the poles, and not an elongation 
there, as, by a singular errour, was once supposed. 

The amount of this flattening, deduced from geo- 
metrical admeasurements, has given it as ^-i-^, that is, 
that the polar diameter of the earth is ¥ ^ less than 
its equatorial diameter: making a difference of some- 
thing more than 26 miles. These measures are given, 
mathematically, by the movements of the moon, 
with much more precision than they can be determined 
by measures taken upon the earth's surface; for it is 



170 THE EARTH. 

a truth, however singular it may appear, that the very 
form of our globe is disclosed by certain inequalities 
in the movements of the moon. These inequalities 
would not exist, if the earth were a perfect sphere; 
and we are able, therefore, to determine the quantity 
of the earth's ellipticity, by mathematical results, de- 
duced from observations of these lunar movements. 

Gravitation has also furnished us the means of de- 
termining the oscillations of the pendulum; which 
vary, at different points of the earth's surface, with 
the variation of the force of gravity. The following 
are the precise measures of the earth, in miles and 
parts of miles: Miles . Decimal , 

Diameter, at the equator, 7925 .648 

Diameter, through the poles, 7899 .170 

Mean diameter, 7912 .409 

Difference between the polar and 

equatorial diameters, 26 .478 

This last is the exact quantity by which the figure 
of the earth deviates from a perfect globe, in its form. 

The length of a terrestrial degree, determined by 
trigonometrical admeasurement, at the point midway 
between the equator and the poles of the. earth, has been 
found to be 364492^ feet. That degree which re- 
sults from the arch of the meridian which crosses 
France, from Dunkirk to Barcelona, and which has 
since been extended to the island of Formentera, con- 
verted into the itinerary measures of different coun- 
tries, gives the following results: 

The geographical league of France is of 25 to a de- 
gree; the marine league is 20; each marine league is 
equal to three minutes of a terrestrial degree; | of a 
league is equal to one mile, or one minute of the equa- 
tor — this is the mile of England, or of Italy: the league 
of Spain, or of Holland, and the mile of Germany, are 
of 15; that of Sweden, of 12; that of Hungary of 
10; and the Werst of Russia of 90, to the degree. 



THE EARTH. 171 

The entire surface of the terrestrial globe contains 
25,790,440 square leagues, of which about three 
fourths are covered by the sea; and of the rest, 
scarcely one half is inhabited. 

In this sketch of the dimensions of the earth we 
have not spoken of the inequalities of its surface; and 
for the reason that the highest mountains may be con- 
sidered as insensible, relatively to the earth's entire 
volume; so that the surface of the globe, in despite of 
the innumerable asperities which it presents, may be 
comparatively regarded as infinitely more uniform than 
the surface of an orange. 

MOVEMENT OP THE EARTH. 

The sphericity of the earth being established, and 
its dimensions known, we may now occupy ourselves 
with its motions. We will first demonstrate that it 
turns upon its own axis, and afterwards that it has, 
beside this, another motion, namely, that of transla- 
tion through space, in an orbit round the sun. 

DIURNAL ROTATION OF THE EARTH. 

All the celestial sphere appears to us to turn entirely 
round the earth once in twenty four hours, and the 
question arises, is this appearance of motion, in the 
starry heavens, real, or is it an illusion'? 

If we compare our earth to the other globes of our 
solar system, and not only to these, but to the infinity 
of fixed stars, which we have seen to be no other than 
suns, at least equal in magnitude to our own, and pro- 
bably constituting centres to so many planetary sys- 
tems, we are readily made sensible that the planet we 
inhabit constitutes but an imperceptible point by the 
side of these enormous masses; and this fact, alone, 
raises a very just doubt, in the mind, whether such an 



172 THE EARTH. 

atom can constitute the centre around which all this 
vast assemblage of immense globes perpetually re- 
volves. But this doubt will give place to astonishment 
when we reflect upon the incredible velocity with 
which these vastly distant bodies must move in order 
to describe, in so short a period of time, such incom- 
mensurable circles. But a still greater difficulty pre- 
sents itself to this theory; which is that, as the velocity 
of these bodies would require to be greater, in pro- 
portion as their distance from the earth is more, it 
would be necessary to admit that the earth attracts 
each of these bodies with a force augmenting in the 
ratio that its distance is increased: a known absurdity, 
since we have seen that the reverse of this is true, in 
the attraction of all bodies. 

Upon this testimony, then, we are forced to reject 
the opinion that the earth is the centre of celestial mo- 
tion; and we must next inquire whether this apparent 
revolution of the heavens may not be an illusion of 
our senses. In instituting this inquiry we are con- 
ducted to the supposition that the real motion is in the 
earth itself: and this once admitted, the phenomenon 
we are considering is all logically and easily explained. 

But, accompanying the globe, as we do, in its revo- 
lution upon its axis, we are not sensible of motion, and 
therefore believe ourselves immoveable, while the stars 
appear to us to move in the contrary direction to that 
in which we are carried. The same is true if we 
place ourselves in a carriage, or on board a ship: 
objects then appear to be carried from us, and with a 
motion more rapid, in proportion as they are nearer 
to us. This illusion is most perfect when the velocity 
is very great; and as the persons in a ship feel not 
the motion by which they are carried forward, so we 
are insensible to the rotary motion of the earth, which 
is vastly more rapid, but which never encounters any 
obstacle or resistance, by which a shock can be pro- 



THE EARTH. 173 

duced, whereby that motion would be rendered sen- 
sible. 

The rotary motion of the earth being thus rendered 
extremely probable by the natural and easy explana- 
tion which it gives of the phenomenon in question* 
and by the evident absurdity of the opposite opinion, 
it now remains for us to prove this motion, directly. 

It has been contended that if the earth turns upon its 
axis, a body projected into the air should, in falling, 
come to the earth behind the point from which it rose: 
that a stone dropped from the top of a tower should 
not fall at the foot of the edifice, because the earth 
would have moved, daring the time of the fall. This 
is an errour: experiments having proved that a pro- 
jected body always partakes of any motion which that 
which projected it may have had. For instance, a 
person in a boat which is moving parallel to the shore, 
may toss into the air a ball, and catch it, as it descends 
although he has moved while the ball was in the air. 
Now, although it will seem to this person that he pro- 
jected this ball vertically into the air, yet viewed from 
the shore, the ball will be seen to have been thrown 
obliquely forward. It is well known that a stone, 
dropped from the top of a ship's mast, when the ship 
is in motion through the water, will fall at the foot of 
the mast, just as if the vessel were at rest: and that a 
bottle filled with water, inverted, and suspended, on 
board a ship which is in motion, with a minute opening, 
for the water to escape, will empty itself, drop after 
drop, into another bottle placed exactly beneath it, al- 
though the vessel might move forward several feet 
during the time that each drop should employ in 
falling. 

But there is another feature to this subject, and from 

that we may draw mathematical proof of the rotary 

motion of the earth. Of two bodies which describe, 

in the same period of time, two circumferences une- 

15 



174 THE EARTH. 

qually elongated from the axis of rotation, that which 
describes the most elongated, and consequently the 
largest, must necessarily move with greater velocity 
than the other. Suppose, then, that from the summit 
of a very elevated tower, a gravitating b.,dy be let 
fall. As the summit of the tower describes a greater 
circumference than the foot, because it is more elonga- 
ted from the axis of rotation, it of necessity moves 
more rapidly; and as it would communicate this motion 
to the body thus let fall, such body would not follow 
the plumb line, in its descent, but would deviate 
towards the east. This has been demonstrated in the 
most convincing manner, by experiments. 

Another demonstration of the rotary motion of the 
earth is borrowed from the transmission of light. But 
before offering this proof, we will establish the fact 
that light is not transmitted instantaneously, but that it 
requires time to pass through space. 

Galileo attempted a solution of this problem, by di- 
rect experiment. To determine it, he prepared two 
lanterns, each furnished with a screen so adjusted as 
that by letting it fall it might instantly intercept all 
light from within the lantern. With one of these he 
repaired to a mountain, while an assistant, with the 
other, ascended a neighbouring height. Galileo had 
instructed his assistant that he should let fall the screen 
of his lantern the instant that the light of Galileo's 
lantern should disappear. He supposed that, if light 
moved progressively, he should detect a lapse of time, 
here, between the instant of shading his own light, 
and the disappearance of the light of his assistant. 
But in this he was deceived: the two lights disappeared 
at the same visible instant; which led him to the erro- 
Jieous conclusion that light is transmitted instantane- 
ously. We proceed to show that this erroneous result 
arose from the fact that the distance included in the 
experiment was not sufficiently great. 



THE EARTH. 



175 



Figure 30, suppose S the sun, T the earth, J Jupi- 
ter at the moment of opposition, and J' Jupiter at the 
Fig. 30. moment of conjunction. If we ob- 

serve two immersions of one of the 
satellites of Jupiter, one at opposi- 
tion and the other at conjunction; 
and afterwards repeat this opera- 
tion, in the inverted order, that is to 
say that, we observe an immersion 
at conjunction, and subsequently one 
at opposition, the time which will 
have elapsed between the first two 
observed immersions will be longer 
than the interval which will sepa- 
rate the last two; the difference 
being 16 m 26 s . Now this difference 
can only arise from the time which is necessary to 
render the immersions visible; in other words, from 
the time which is necessary for light to pass from J' 
to T; and as the operations have been made in in- 
verted order, the difference of 16 m 26 s denotes the 
time in which light passes from J' to T; or, to express 
it differently, 16 m 26 s is the length of time necessary 
for light to traverse the space in question; moving, as 
it does, with a velocity of 192,500 miles in a second 
of time. 

The progressive transmission of light being estab- 




of the rotation of the earth. 

If the earth is immoveable, we should not see the 
heavenly bodies at the moment they arrive upon the 
horizon or the meridian, but only after the lapse of so 
much time as is necessary for the rays of light, from 
them, to reach our eye. 

If, on the contrary, the earth turns upon its axis, 
we should necessarily see these bodies at the instant of 
their arrival, either at the meridian or horizon; for, 



176 THE EARTH. 

by the effect of the movement of rotation the eye 
would be placed upon the line of such rays as had 
issued from these bodies at some previous moment, and 
which, at this instant, had reached the point of space 
occupied by our horizon. 

Now, we do see these bodies at the instant of their 
arrival. The proof of this is, that the observed pas- 
sages of our meridian, of Mars, for example, would 
be more or less hastened, Or more or less retarded, 
according as this planet approaches the earth, or is 
elongated from it, if we saw it not at the instant of its 
arrival there; but no difference of this kind is ever 
observed: it is therefore proved that the earth has a 
rotary motion. 

The earth having a circumference of almost 25,000 
miles, at the equator, it follows that each point of its 
surface, there, passes over the circumference of a cir- 
cle of equal dimensions. The velocity, then, of the 
surface of the earth, at the equator, is equal to that of 
a cannon ball. 

Since the earth, then, has a rotary motion, upon its 
axis, which is now proved, it follows that it is, like all 
other bodies which have this motion, endowed with a 
centrifugal force, of which the intensity, according 
both to experiment and calculation, is in the ratio of 
the squares of the velocities of revolution. From this 
it follows that, under the equator, the centrifugal force 
is at its maximum, while at the poles of the earth it 
is nothing. The intensity of gravity, then, should be 
less under the equator than at the poles; and it has 
been clearly proved that this is so, by the oscillations 
of a pendulum, of the same length, at the equator, 
and at points distant from it. But in considering the 
solution of this problem, we must not forget that the 
difference, obtained in this way, is not all due to the 
agency of centrifugal force; for we have already seen 
that the distance from the surface to the centre of the 



THE EARTH. 



17T 



earth is greater at the equator than at the poles; and 
we have also learned that attraction acts in the inverse 
ratio of the squares of the distances. 

We should, here, perhaps, add what has been, and 
still is, among astronomers, their explanation of the 
cause of the peculiar shape of the earth, namely, its 
enlargement, at the equator, and depression at the 
poles. 

The earth, like all the other planets, we suppose, 
with the Geologists, to have been primitively fluid; at 
least this is an opinion which observations and theory 
concur in affirming, and which is generally admitted, 
at the present day. This assumed, suppose given to 
the earth a rotary motion round A B, fig. 31. The 
Fig. 31. particles of matter situated in the 

right line A B, that is to say, upon 
the line of the poles, are not en- 
dowed with any centrifugal force, 
and consequently lose nothing of 
their weight. On the contrary, 
those particles situated in the line B 
C are subject to the influence of cen- 
trifugal force, which paralyzes, in 
part, the force of gravity, thereby 
rendering them lighter: a greater quantity of them, 
therefore, is necessary for maintaining the equilibrium. 
Fig- 32, It is easy to suggest an 

« experiment which will show 

.„ n ".a that the velocity of rotary mo- 

wp'iinr ^___I3DBBM t j on w ^ produce a spheroid, 
flattened like that of the earth. 
Suppose two strips of paste- 
board, or other flexible sub- 
stance, bent into circles, and 
mounted upon an axis, as in 
fig. 32, so as to revolve with 
it. If these be made, by means of the crank, G, to 
15* 





178 THE EARTH. 

revolve slowly, they will experience no change of 
form; but if a rapid rotary motion be imparted to 
them, their poles will become depressed, and the cir- 
cles will extend themselves upon the sides, or equator. 

ANNUAL MOTION OF THE EARTH. 

We have seen that the earth revolves upon its axis, 
in 24 hours, and that the apparent revolution of the 
celestial sphere is only the effect of an illusion, pro- 
duced by this motion of the earth. It remains for us 
now to inquire whether the annual motion of the sun 
is real, or whether it is only an appearance, caused by 
the displacement of the earth: for we have now 
learned to distrust the first evidences of our senses, in 
cases like this. 

But we will first describe this motion. If we care- 
fully observe the sun, each day, we shall see that it 
advances, every twenty-four hours, about 1° towards 
the east. Now, this 1° corresponds to four^minutes * 
of time; consequently the sun arrives four minutes 
later, each day, in the plane of the meridian; and in 
ninety days this daily sum amounts to six hours of 
time, which the sun will be later upon the meridian 
than the star with which it at first arrived there. At 
the end of a little more than 180 days, both the sun and 
this star will be in the plane of the meridian, at the 
same moment, but upon opposite sides of the earth, 
the one being in the superiour, and the other in the 
inferiour meridian. Finally, after 365? days, or one 
year, the sun, and the star with which it was first ob- 
served, will again be found on the meridian together, 
and upon the same side of the earth. The line which 
the sun may be said thus to have traced, is the eclip- 
tick, of which the plane is inclined to the equator, 23° 
28' The most elevated points of the ecliptick have 
received the name of solstices, because the sun seems 



THE EARTH. 179 

to stop, in its progress, from the equator, at these 
points: and the equinoxes, that is to say, the epochs 
at which the days and nights are equal, take place 
when the sun is in the plane of the equator; which 
consequently occurs twice in each year. Such is the 
path which the sun appears to follow in the course of 
a year. But is this apparent motion real, or is it not 
rather the earth which traverses the ecliptick, and 
gives rise to the appearances which we witness? 

In answer to this question, if we resort to the induc- 
tions of analogy, we see, at once, that it is much more 
rational to admit that the earth, which we have already 
learned only lacks a revolutionary motion, in an orbit, 
to entitle it to place and rank among the planets, is 
really endowed with such a motion, than to suppose 
that the sun, with all its attendant array of planets, is 
in rapid revolution round the earth, in defiance of the 
laws of gravitation. But the probability of the move- 
ment of translation of the earth, already so great, at- 
tains the last degree of certainty when we deduce 
form observed phenomena the natural explanations 
and demonstrations which they afford, and which are 
such as wholly to remove all doubts upon the question. 

Upon the supposition of the earth's immobility, in 
what possible manner can we account for the fact that 
the planets sometimes become stationary, and at 
others have a retrograde motion? And what is more 
easy, or natural than this explanation, upon the con- 
trary hypothesis? 

We have seen, in speaking of the planets, that 
these bodies appear to move, sometimes from west to 
east, sometimes from east to west, while at other times 
they appear stationary among the stars. Such are 
the phenomena. Now, suppose that the earth moves 
in the ecliptick, and let us examine into the results, 
upon this hypothesis. Figure 33, suppose S the sun, 
T the earth, and M Mars, for example. The earth, 



180 



THE EARTH. 



moving more rapidly than Mars, would be at T' when 
this planet would have advanced only to M'. Mars 
Fig. 33. would then, by reason of 

the illusion of which we 
have already spoken, ap- 
pear to have retrograded 
towards M. But when the 
\ earth has arrived at T", the 
\ path it will have followed 
\ so inclines, in reference to 
that of Mars, that this lat- 
ter planet will appear to be 
stationary. Finally, when 
the earth shall have reach- 
ed T'", the relative posi- 
tions of the two planets 
will be such as to repre- 
sent Mars as having again 
resumed its appropriate, 
progressive motion. 

Such is, upon the hy- 
pothesis of the motion of 
the earth, the natural and 
easy explanation of the phenomena of the stationary 
and retrograde appearances of the planets — an expla- 
nation which is sought, in vain, in every other system. 
Bradley, in endeavouring to, determine the annual 
parallax of the fixed stars, discovered that they are 
not immoveable, but that they appear to describe, 
during the time that the earth employs in passing 
round the ecliptick, those which are in the plane of 
the terrestrial orbit, right lines; those which are in 
the plane perpendicular to this orbit, circles; and those 
which are in the intermediate planes, ellipses, more or 
less elongated, according as they are more or less 
near to the one or the other of these positions. This is 
the phenomenon of the aberration of light; and it has 








THE EARTH. 181 

furnished us with a new demonstration of the motion 
of the earth, through space. 

Let us here recall to mind that light does not reach 
us instantaneously, from the stars, but that time is con- 
sumed in its transmission to us. This premised, sup- 
Fio-. 34. P ose C A, fig. 34, a luminous ray falling 
c perpendicularly upon the line B D. If 

the eye is at A, and in repose, it will see 
the object from which the ray proceeds, 
in the direction A C; but if the eye is in 
motion from B towards A, and if light is 
propagated with a velocity which shall be 
to that of the eye as the distance C A is 
& to B A, then light will move from C to A 
while the eye moves from B to A. Now, every par- 
ticle of light which renders the object visible, on the 
eye's arrival at A, is in C when the eye is at B. Join, 
then, the two points B and C, and suppose that the 
line C B is a tube, inclined to the line B D, and of 
such a diameter that only a single particle of light 
could find admission. It is evident that the particle of 
light from C, which would render the object visible 
when the eye, in its progressive motion, shall have ar- 
rived at A, passes through the tube B C, which ac- 
companied the eye in its movement, and preserved its 
inclination. Now, if the particle of light reached the 
eye by passing through the tube B C, then the eye 
would see the object in the direction of this tube. If, 
instead of supposing the tube thus small we merely 
make this the axis of a larger one: the particle of 
light would then always pass through this axis, if the 
proper inclination of the tube were maintained, pro- 
ducing, of course, the same result. If we suppose 
the eye to move from D to A, the same results would 
follow; only that the tube, C D, would be inclined in 
the opposite direction. 



182 THE EARTH. 

It follows, from this, that if the earth moves, we see 
not the stars in their true position, but a little in ad- 
vance of that position; and the difference between 
their real and apparent place is, to the sine of their 
visible inclination to the plane of the ecliptick, as the 
velocity of the earth is to that of light. 

It is now easy to conceive that, the movement of 
the earth admitted, the fixed stars should present just 
the phenomenon observed by Bradley; and the expla- 
nation which we have given of this phenomenon, 
otherwise inexplicable, constitutes the most unanswer- 
able proof of the revolution of our globe. 

The earth, then, is no longer to us the immoveable 
centre, around which the whole universe gravitates; 
but it is found to be only a small planet of the solar sys- 
tem, obedient, like all the rest, to the laws of attrac- 
tion. Its distance from the sun is about 95,000,000 
of miles. Its annual revolution is accomplished in 
365 d 5 h 48 m 49 s , which is called its tropical year; but 
the time which it requires to complete its annual revo- 
lution, assuming a given fixed star as the point of de- 
parture and of arrival, is 365 d 6 h 9 m 12 s , and this is 
designated the sidereal year. The rotation of the earth 
upon its axis is accomplished in 24 hours, which con- 
stitute the length of the natural day. The mean dia- 
meter of the earth is 7912 t Yq- 9 q- miles. A point upon 
the equator moves, by means of the earth's rotation, 
nearly T \ of a league per second, while the earth is 
translated through space, in its orbit, about seven 
league's per second; which is but little more than half 
the velocity of Mercury, while it exceeds that of Ura- 
nus by more than four to one. The diameter of the 
earth's orbit is about 180,000,000 of miles. Farther 
details, of this kind, are rendered unnecessary here, 
by the tables which we have inserted in a previous 
part of this book. 



LUNAR AND TERRESTRIAL INEQUALITIES. 183 

CHAPTER TENTH. 

OP THE SECULAR AND PERIODICK INEQUALITIES. 

Bodies, as wc have seen, mutually attract each 
other, according to certain laws, and in consequence 
of this, all the globes of our solar system should be 
disturbed, each by every other, and thereby subjected 
to an almost infinite variety of perturbations. This 
is precisely what astronomical research has shown 
to exist; and it constitutes one of the most unanswer- 
able evidences of the accuracy with which the law of 
gravity has been determined. There is not one of 
these derangements, or inequalities, however minute, 
of which this law does not render the most rigorous 
appreciation. 

The irregularities which the planets and their satel- 
lites experience, in their movements, have received 
the name of inequalities. These have been divided 
into secular, and periodick inequalities: not that the 
first are, in fact, less strictly periodick than the last, 
but only because these are not perfected except after 
the lapse of vast periods of time, while the last recur 
at much shorter intervals. 

Nevertheless, these derangements are all limited, 
and have bounds set to them, which they never can 
exceed. The curves described by the planets may be 
more or less irregular, may approach to, and recede 
from the circular form, but the distance from the sun 
is constant, and invariable: the angle of inclination of 
the axis to the orbit may experience variations; but 
these can never extend beyond certain limits. 

We propose, here, to speak only of the most re- 
markable inequalities of the moon and the earth. 

LUNAR AND TERRESTRIAL INEQUALITIES. 

When the moon is in conjunction, that is to say, 
when, by reason of its movement of revolution, it is 



184 LUNAR AND TERRESTRIAL INEQUALITIES. 

placed between the sun and the earth, it is of course 
nearer the first of these bodies than when in the oppo- 
site situation, and the solar attraction acting with in- 
creased energy, in this case, the distance of the moon, 
from the earth, is thereby augmented. On the con- 
trary, when the moon is in opposition, that is to say, 
when the earth is between the moon and the sun, this 
last body, attracting the earth more strongly, elongates 
it from its satellite, the moon. In the quadratures the 
case is still different: the action of the sun, then, leaves 
that of the earth to preponderate. Now the imme- 
diate effect of these derangements is to exercise a con- 
trol over the velocity of the moon; and by them its 
movement is retarded, from the conjunction to the first 
quadrature, and accelerated from this quadrature to the 
point of opposition. The velocity then diminishes to 
the second quadrature, and is afterwards augmented 
from this point to that of conjunction. These inequali- 
ties are called variations. 

Nevertheless, as the moon accompanies the earth, 
in its movement round the sun, and as the earth, in 
this movement, sometimes approaches the sun, and 
sometimes recedes from it, it is readily seen that this 
variation in the distances must produce modifications 
in the phenomena which we have described. This 
new species of inequalities has received the name of 
annual equation. 

We have already seen, in treating of the moon, 
that its nodes move, upon the ecliptick, from east to 
west, at the rate of 19° .3286 per year, which would 
constitute an entire revolution in 18 years, lh months, 
about; or more exactly, in 6788 days .54019. This 
motion of the nodes of the lunar orbit, and the varia- 
tions of its inclination to the ecliptick, are due to the 
action of the sun; for, when the moon, in its motion 
of revolution round the earth, approaches the plane of 
the ecliptick, the force of the sun's attraction hastens 



LUNAR AND TERRESTRIAL INEQUALITIES. 185 

this approach, and thus anticipates the moment when 
it would cut the plane of the ecliptick. From this 
arises the retrograde movement of the nodes; and the 
change of the inclination of the moon's orbit to the 
ecliptick. 

The attractive force of the earth, upon the moon, 
varies in intensity, according as this last is in apogee 
or perigee, and consequently giving more or less in- 
fluence to the solar attraction. From this cause arise 
elongations and contractions in the lunar orbit. These 
inequalities are called ejections. 

But the most remarkable of these inequalities is tlie 
precession of the equinoxes. The sun does not cut the 
celestial equator, every year, at the same point. If on 
a certain day it cuts it at a given point, the same day 
of the following year it cuts it at a point situated 
50" .103 to the west of the first, thus arriving at the 
equinox 20 m 23 s before having completed an entire 
revolution of the heavens, from one fixed star to the 
same again. Thus the tropical year, or the true year 
of the seasons, is shorter than the sidereal year. The 
precession of the equinoxes is an effect of the solar at- 
traction, that acts with more intensity upon the in- 
creased quantity of matter at the equator, which it 
tends to draw into the plane of the ecliptick, but which 
maintains its inclination by the effect of its motion of 
rotation. Retrograding each year, in this manner, 
towards the west, by the quantity 50" .103, the equi- 
noxes make an entire revolution in 25,867 years. 
Thus Aries, the Ram, <p, which formerly corres- 
ponded to the vernal equinox, is now 30° west of it; 
although, by the conventional language of astrono- 
mers, it is always spoken of as denoting this equinox. 

The retrograde movement of the equinoctial points 
causes the axis of the earth to describe, by virtue of a 
conical movement, a small circle, of which the diame- 
ter is equal to twice its inclination to the ecliptick, 
16 



186 LUNAR AND TERRESTRIAL INEQUALITIES. 



that is 46° 56'. Suppose, fig. 35, N Z S V L the 

earth; its axis prolonged to the stars, and terminating 

at A, the actual north pole of the heavens, which is 

Fig. 35. 




J 



vertical to N, the north pole of the earth: suppose, 
farther, EOQ the equator, T Z the tropick of Can- 
cer, and V Y3 tnat °f Capricorn: V O Z the ecliptick, 
and B O its axis, which should be considered as im- 
moveable, because the ecliptick always passes through 
the same stars. But, as the equinoctial points retro- 
grade in this plane, the axis of the earth, S O N, is in 
motion, upon its centre, O, as a pivot, in such a man- 
ner as to describe the double cone NOn, and S O 5, 
around that of the ecliptick B O, in the time which the 
equinoctial points move round this plane, that is, in 
25,867 years; and in this long interval the north pole 
of the axis of the earth describes the circle A B C D A 
among the stars, round the pole of the ecliptick, which 
remains immoveable, in the centre of the circle. The 
axis of the earth being inclined 23° 28' to that of the 
ecliptick, the circle A B C D A, described by the north 
pole of the axis of the earth, prolonged to A, is near 



LUNAR AND TERRESTRIAL INEQUALITIES. 187 

46° 56', or double the inclination of the axis of the 
earth. Consequently, the point A, which is at present 
the north pole of the heavens, and very near to a star 
of the second magnitude, in the end of the tail of the Lit- 
tle Bear, will be abandoned by the earth's axis which, 
retrograding one degree in 71 § years, will be directed 
towards the star at the point B, in 6,447 1 years; and, 
in double that time, or 12,8954 to the star, or point C, 
which will then be the north pole of the heavens. The 
present position of the equator, EOQ will then be 
changed to e O q; the tropick of Cancer from T Z to 
V °5, and that of Capricorn from V 1$, to t Z. The 
sun, in the part of the heavens where it now is upon 
the terrestrial tropick of Capricorn, and produces the 
shortest days and the longest nights, in the northern 
hemisphere, will then be upon the terrestrial tropick 
of Cancer, giving the longest days and the shortest 
nights. This effect cannot be fully realized until the 
lapse of 12,895 years, reckoning from the point C; or 
rather, if we count from the point of departure A, after 
25,867 years, the period necessary for the north pole 
to make one complete revolution, and return to the point 
of the heavens vertical to that which it now occupies. 

Bradley having discovered the aberration of light, 
was pursuing farther observations for its verification 
when he perceived that the axis of the earth inclined, 
sometimes more and at others less to the ecliptick, cau- 
sing the like variations in the inclination of the planes 
of the ecliptick and the equator, and describing, around 
the mean pole, as a centre, a small ellipse of which the 
major axis subtends an arch of the celestial sphere, 
of 20" .153, and the minor axis one of 15" .001. 
This ellipse is described in the exact period of the 
lunar cycle, that is to say, in very nearly eighteen 
years and seven months. The period of nutation 
being precisely that of the movement of the nodes of 
the moon, these two phenomena are necessarily allied. 



188 LUNAR AND TERRESTRIAL INEQUALITIES. 

It is, indeed, the attraction of the moon, acting with 
more intensity upon the equatorial regions than upon 
the poles, which causes this phenomenon of nutation. 

Finally, beside the two inequalities which we have 
described, in the movements of the earth, and which 
are the two principal ones to which this planet is sub- 
jected, there is one other, of great importance, and 
which is the result of all the attractions which the pla- 
nets collectively exert upon it. This is the gradual 
displacement of the plane of the ecliptick in the hea- 
vens, and the diminution of its inclination to the equa- 
tor, by a quantity equal, or very nearly so, to 52" .1154 
per century; being about one hundredth of the preces- 
sion, namely, h" per year, 1' in 115 years, and 1° in 
6,900 years. 

This change of obliquity in the inclination of the 
equator to the ecliptick, is confirmed alike by the obser- 
vations of ancient, compared with modern astronomers, 
and by calculations. Modern astronomers have assu- 
red themselves of its truth by comparing the present 
positions of stars, relatively to the ecliptick, with 
those which they occupied, in relation to it, in the times 
of the earliest observations that we possess. Observa- 
tions have farther shown that those stars which, ac- 
cording to ancient records of their positions, were situ- 
ated to the north of the ecliptick, near the summer sol- 
stice, are now more advanced towards the north, and 
more elongated from this plane; while some, that were 
farther south, are now found comprised in the plane, 
and others, still, have passed to the north of it. 

It is this motion, so well demonstrated, that has been 
so often, in books of science, as well as elsewhere, as- 
serted to be perpetually progressive, and by the agency 
of which the equator and the ecliptick it is said have 
formerly been made to coincide, as they will again 
hereafter do. But Laplace has clearly demonstrated 
that this diminution of the obliquity of the ecliptick can- 



OF COMETS. 189 

not be continuous; but that, after a certain period, this 
motion becomes first retarded, and finally entirely ar- 
rested; after which a motion in the opposite direction 
ensues. In short, this movement he has shown to be 
a fixed and constant vibration, perpetually progressing 
either one way or the other, thus fully balancing itself, 
and never exceeding a fixed limit of between one and 
three degrees. 



CHAPTER ELEVENTH. 

OP COMETS. 

We have next to devote our attention to a class of bo- 
dies which have given rise to a great variety of disso- 
nant opinions. These are Comets, a species of celes- 
tial bodies of which the appearance of one has, in all 
former time, struck the mass of mankind with astonish- 
ment and fear. 

But, before entering upon an examination of these 
bodies, some definitions of them, and of the several 
parts of which they are constituted, are necessary. 
As a farther aid to the pupil, in understanding the defi- 
nitions which follow, the drawing, fig. 36, exhibiting 
Fig. 36. 



Comet of 1680. 
the appearance of the comet of 1680, is given: but it 
must not be forgotten that these bodies are greatly di- 
versified, and bear little resemblance to each other, 
while the same comet seldom maintains an entirely un- 
16* 



190 OF COMETS. 

changed appearance, even for so long a period as a 
single night. 

The word Comet signifies a hairy star, and was ori- 
ginally adopted because it was supposed that all comets 
had this appearance; and although later observations 
have shown that this is not so, particularly as regards 
the tail, yet the name, having been so long in use, is 
retained. 

The central part, which varies both in size and 
brightness, is called the nucleus. 

The cloudy or hazy appearance, which in many 
comets surrounds the nucleus, is called the envelope. 

The luminous trains, by which the greater part of 
comets are accompanied, were formerly designated 
beard, or tail, according as they preceded or followed 
the comet, in its motion. Now, however, the term 
tail is applied to all these appendages, in whatever di- 
rection they may project from the nucleus. 

The nucleus and its envelope are collectively called 
the head of the comet. 

Modern astronomers do not class among the number 
of essentially distinctive characteristicks of comets, ei- 
ther the envelope or the tail. According to them, any 
heavenly body is a comet which has a proper motion, 
that is, has a motion of its own, and passes round the 
sun in an ellipse of so great eccentricity that it ceases 
to he visible from the earth, during some part of its 
revolution. 

The observations simultaneously and daily made 
upon different parts of the earth, at great distances 
from each other, and the participation of comets in the 
general revolution of the celestial sphere, permit of no 
farther doubt that comets are permanent, celestial bo- 
dies, and not meteors, engendered in our atmosphere, 
as was formerly supposed. 

For a long period it was thought that comets fol- 
lowed no regular movement; that they were not sub- 



OF COMETS. 191 

ject to the laws which regulate and govern the other 
celestial bodies, but that they wandered from system to 
system, through the immensity of space. But since 
the discoveries of Kepler investigations have been made 
to know if these bodies are exempt from his laws, and 
also to determine their orbits. For this last determi- 
nation it is sufficient to know three positions of the co- 
met, 1st. The longitude and the inclination of the node; 
2d. The longitude of the perihelion, and 3d. The peri- 
helion distance: and to these data must be added whe- 
ther the motion be direct or retrograde, in as much as 
these bodies, unlike the planets, move in every direc- 
tion. By these means the curves which several co- 
mets describe have been determined; which has disclo- 
sed the fact that they move in ellipses of very great 
eccentricity, of which the sun occupies one of the foci. 
Nevertheless, comets having been anciently but little 
observed, and that very imperfectly, most of the ele- 
ments necessary to the determination of their identity 
are wanting, which renders it very difficult to assign, 
to many of them, their period of return. It is not, in- 
deed, impossible but some of them move in parabolick 
orbits, that is, in curves open at the end farthest from 
the sun, in which case, of course, they could never 
return towards the sun; but, while #the possibility of 
this is admitted, because no law is known to forbid it, 
yet there is no good reason to suppose such a case 
ever existed; while it would furnish evidence of imper- 
fection and derangement, in the celestial mechanism, 
of which no parallel is to be found. 

As the physical circumstances of form, size, light, 
&c. of comets often materially vary, even in periods 
of only a few days, they can never be recognised, by 
any of these characteristicks, as the same body which 
was previously seen. For the purposes of such recog- 
nition the parabolick elements of the orbit are alone 
depended upon. But will the identity of two comets, 



192 OF COMETS. 

appearing at different epochs, be always infallibly de- 
termined by this means'? 

If the parabolick elements of two comets are differ- 
ent, we are still far from being certain that the two 
bodies are not the same; because, in passing near a 
planet, a comet may be so deranged in its course that 
its curve, or orbit, after such derangement, may be 
entirely changed. On the contrary, if the orbits of 
the two comets which are compared, have very nearly 
the same parabolick elements, their identity is thus 
rendered highly probable. Still it is not impossible 
that two different comets may describe two curves, 
similar in form and position; but when we consider 
how many different elements and circumstances enter 
into the similitude, we are not easily dissuaded from 
the belief that two comets, which appear at different 
periods, with the same elements, are one and the same 
body. 

For the purpose of furnishing to astronomers, as far 
as possible, the means of determining, when a comet 
appears, whether it is one that has been previously ob- 
served, there exist catalogues of all the comets, in 
which are regularly recorded all their parabolick ele- 
ments, so far as they are known. These elements are 
still very few in aumber, good observations of comets 
being entirely modern. Consequently there are, to 
the present time, but three comets whose regular, pe- 
riodick return is known. 

comet op 1759. 

The astronomer Halley having calculated, in 1682, 
the parabolick element of a comet which appeared in 
that year, was struck with the analogy which he found 
to exist between his results and those obtained by Kep- 
ler, for a comet which appeared in 1607. He next 
recurred to observations still more ancient, and there 



OP COMETS. 193 

discovered that the elements of a comet observed by 
Apian, in 1531, bore a very strong resemblance to his 
own. His inference was that this was the same comet, 
which had re-appcared at very nearly equal intervals 
of time, that is to say, about seventy-six years; and 
from this data he hazarded the prediction that this co- 
met would return towards the end of the year 1758, or 
at the commencement of 1759. But Clairault having 
calculated, with great patience and labour, that the 
comet would be retarded 618 days by the attraction of 
Jupiter and Saturn, it finally arrived at its perihelion 
on the 12th of March, 1758. This comet is the first 
one of which the return was predicted, and that pre- 
diction verified. 

Damoiseau, one of the Bureau of Longitude, of 
France, calculated the period of this body's return, in 
1835, and fixed its perihelion passage on the fourth of 
November. Pontecoulant, another eminent French 
mathematician, who made the like calculations, first 
fixed on the seventh of November as the period of its 
passage. He afterwards, by a more complete and ri- 
gid valuation of the disturbing action of the earth, and 
by the substitution of a new value for the mass of Ju- 
piter, carried forward the period of its passage to the 
thirteenth of the month. The actual passage took 
place, according to observation, on the sixteenth. This 
difference of only three days, in a period of more than 
seventy-six years, implies an accuracy which has asto- 
nished even astronomers. When it is considered how 
many causes of disturbance all comets are subject to, 
and how many of these, in the case of comets of so 
long a period, may be wholly unknown to us, such as 
the action of other planets, beyond uranus, of which 
we have no knowledge, or the action of other comets 
there, equally unknown to us; and above all, when 
we reflect that a knowledge of the exact weight of the 
sun, and of every planet in our solar system is abso- 



— 



194 OF COMETS. 

lutely necessary to an accurate solution of the question 
here under examinaton, the only wonder is that a re- 
sult so nearly accurate should have been obtained.* 

comet of 1770. 

This comet was discovered by Messier, in the month 
-of June, 1770; and Lexell found that it had, in five 
years and a half, described an ellipse of which the 
transverse axis was only three times the diame- 
ter of the earth's orbit. Great astonishment was ma- 
nifested at this, namely, that a comet which, with so 
short a revolution, might often have been seen, and 
still was not perceived before the discovery of Messier; 
and this astonishment was redoubled when, at the end 
of five and a half years, it failed to return and again 
traverse the elliptical orbit assigned to it by Lexell. 
The causes of this mysterious disappearance, which 
gave rise to so many sarcasms, good and bad, at the 
expense of the astronomers, for their lost comet, are 
now fully and clearly known; although at that period 
they were not. These causes are found to originate 
in the law of universal attraction, of the truth of 
which they have furnished renewed confirmation. 
Lexell had remarked that in 1767 and 1779 this comet 
approached very near to Jupiter, whose powerful at- 
traction had diminished, in 1767, the perihelion dis- 
tance of its orbit, so as to bring the comet within view 
of the earth in 1770, by withdrawing it from its pre- 
vious orbit, where it was invisible, from the earth; and 
afterwards, namely, in 1779, the influence of the same 
planet, acting in an opposite direction upon the comet, 
increased its perihelion distance to such a degree as to 
carry the comet again beyond our sight. Laplace 
submitted these suggestions to analysis, and demonstra- 

* For more minute details of the difficulties which surround this 
question, see appendix. 



OP COMETS. 



195 



ted that such had been the result of the attraction of 
Jupiter, upon this comet, at different periods. The ap- 
pearance, then, of this comet, for the first and last 
time, in 1770, is fully explained, when we add that its 
return to its perihelion in 1776, took place during day- 
light, which alone prevented the inhabitants of the 
earth enjoying a second view of a body whose strange 
disappearance caused so much remark. 

COMET OF A SHORT PERIOD. 

This comet was discovered at Marseilles, by Pons, 
on the 26th of November, 1818. Its parabolick ele- 
ments, which were determined by Bouvard, caused it 
to be recognised as the same body which was seen in 
1805, and Encke demonstrated that it completed a 
revolution in its orbit, in 1,200 days, or about 3^ 
years; and records of previous appearances of this 
comet confirm these calculations. 

THE COMET OF SIX YEARS AND THREE QUARTERS. 



This comet was discovered by Biela, at Johannis- 
burgh, on the 27th of February, 1826. Gambart, 
who discovered it at Marseilles a few days after, de- 
termined its parabolick elements; and by comparing 
these with previous records, he learned that the same 
body had been observed as early as 1772, and again 
in 1805. 

This is the comet which so greatly frightened many 
of the people of both Europe and America, because 
some one had announced that it would come in contact 
with the earth, on its return to the sun, in 1832. It 
is true that this comet did on the 29th of October, 
cross the earth's orbit, or path, at the point where the 
earth arrived on the 30th of November, following. 
By recurring to the mean velocity of the earth, in its 



196 OF COMETS. 

orbit, which is about 1,620,000 miles per day, we 
shall see that when this comet passed through the 
earth's orbit, the earth itself was at a distance of 
more than 48,000,000 of miles. In 1805 the same 
comet passed within about one tenth part of the dis- 
tance from the earth, that it did in 1832, but without 
the slightest visible effect, of any kind, upon our 
globe. We shall speak, hereafter, of the possibility 
of a collision between the earth and a comet. 

PHYSICAL CONSTITUTION OP C03IETS. 

This branch of cometary astronomy is not far ad- 
vanced; yet we will proceed to make known the state 
of the science in regard to the envelope, the nucleus 
and the tail of comets. Among all those comets which 
have been observed, to the present period of time, a 
great number have no perceptible tail; several present 
no apparent nucleus; but all exhibit that cloudy or 
hazy appearance which is known by the name of en- 
velope. 

The matter composing this envelope is so rare and 
transparent that the most feeble light is not obstructed 
by it; and consequently the smallest stars are not ob- 
scured by this part of a comet, when passing before 
them. 

In such comets as have a nucleus, those parts of 
the envelope which are nearest to it are usually rare, 
very transparent, and but feebly luminous. But, at 
a certain distance from the nucleus, the envelope be- 
comes all at once much enlightened, so as to form a 
luminous ring round the comet. Sometimes two, and 
even three of these rings are visible, concentrically 
arrayed, with dark spaces between them. These rings 
are, of course, an optical illusion, that which appears 
a ring, in projection, being, in reality, a spherical 
envelope. 



OP COMETS. 197 

When the comet has a tail this ring has the ap- 
pearance of a semicircle, of which the convexity is 
turned towards the sun — when the tail is opposite the 
sun — and from the extremities of which issue the 
most luminous portions of the tail. 

The ring of the comet of 1811, was 10,000 leagues 
in thickness; and its distance from the nucleus was 
about 12,000 leagues. The comets of 1807 and of 
1799 had each two rings, of 12,000 and of 8,000 
leagues in thickness. 

We have said that there exist comets without appa- 
rent, or visible nucleus: these we cannot doubt are 
mere globes of gaseous matter; but there are many 
which present nucleuses very similar to planets, both 
in form, and in the appearance of their light. This 
was the case with Halley's comet, or the comet -of 
1759, as seen at Buffalo, at 7 o'clock on the evening 
of the 21st of October, 1835. Observed, on that oc- 
casion, with the aid of an acromatick telescope, under 
a magnifying power of sixty, "the nucleus was very 
distinct, and well defined; its diameter three fifths 
the mean diameter of Jupiter, under the same power, 
and of a colour midway between Mars and Jupiter."* 
At no other time, however, did the observer of this 
detect the same, or nearly the same, appearances in 
this comet, during its approach to the sun, in 1835. 
These nucleuses are usually very small; yet occasion- 
ally there is one of great size; and they have been 
measured of from eleven, to 1089 leagues in diameter.. 

Some astronomers have attempted to prove, sup- 
porting their positions by observations, that the nu- 
cleuses of comets are always transparent, or, in other 
words, that all comets are only simple masses of va- 
pour, or gaseous matter. But, not only do the obser- 
vations cited in support of this opinion prove nothing 



* Observer's notes. 

17 



198 OF COxMETS. 

in favour of the positive terms in whieh the opinion 
itself is expressed, but they are in formal opposition to 
other observations, no less worthy of confidence. 
From a careful examination of all the observations 
extant upon this subject, it seems most probable that 
there are some comets which have actually no nucleus; 
others, of which the nucleus is perhaps wholly, or in 
part, transparent; and others, still, which are very 
brilliant, of which the nucleus is probably solid and 
opaque. 

As to the tails of comets, science possesses very 
little certain data respecting them. 

These luminous trains are perhaps most frequently 
placed nearly behind the comet, as seen from the sun, 
but they almost always deviate some degrees from this 
right line, and occasionally they have been seen 
forming a right angle with it. The tail, when nearly 
behind the comet, as seen from the sun, usually in- 
clines somewhat towards the region which the comet 
is leaving. This has been supposed, by many, to be 
caused by the resistance of a very rare, invisible me- 
dium, called ether, which has been imagined to occupy 
the regions of space, beyond our atmosphere, and 
which is supposed to act more strongly upon the light 
matter of the tail than upon the more solid parts of 
the head of a comet. It has been farther urged, in 
favour of this hypothesis, or of its probability, that 
the inclination of these tails, which has been men- 
tioned, is greater, in proportion as the distance from 
the head of the comet increases. But we must not 
forget that this theoiy is strongly discredited, if not 
absolutely disproved, by the fact that comets' tails 
have formed almost every possible angle with the 
right. line joining the sun and comet, and that some 
comets, as that of 1824, have had one appendage of 
this kind tending from the sun, and another projected 
directly toivards that luminary. There have also been, 



OP COMETS. 199 

as we shall soon see, comets with a still greater num- 
ber of tails, and of course forming very different an- 
gles with this line.* 

The tail of a comet enlarges in width, in proportion 
to the increase of distance from the head; and the 
centre of its width, throughout its entire length, is 
usually much darker than the rest; and this has often 
been supposed to be the shadow cast by the head of 
the comet. But this explanation is manifestly de- 
fective, as it is wholly inapplicable to all those tails 
which are not opposite to the sun. This phenomenon 
is far better explained by supposing the tail of a co- 
met a hollow cone, of which the envelope has a cer- 
tain thickness. Now, if this is so, we readily see 
that the eye would encounter more of the nebulous 
particles of which this envelope is formed, near the 
borders of the cone, than in its centre; and as the in- 
tensity of light is in the ratio of the number of these 
particles, the existence of the two bands of light, 
upon the borders, with an interval between them, com- 
paratively obscure, is easily explained. 

As we have said above, comets have sometimes had 
several tails. That of 1744, for example, on the 7th 
and 8th of March, had no less than six of these ap- 
pendages, perfectly distinct, and separated from each 
other by spaces, as dark as other portions of the hea- 
vens. They were each about four degrees broad, 
and were spread out like a fan. 

The tails of comets are sometimes of enormous di- 
mensions. Some of them, as those of 1680, 1769, 
and 1618, were so long that when the comets were in 
the zenith their tails extended quite to the horizon. 
The tail of the comet of 1680, was computed to have 
extended more than 96,000,000 of miles; a distance 
greater than that of the earth from the sun! 

* For a more full and detailed examination of this subject, and 
the evidences connected with it, see appendix. 



200 OF COMETS. 

But what are these tails of comets'? How are they 
produced? What are the causes which so constantly 
modify and change their forms'? What are those 
which control the formation of the envelope, and of 
the concentrick rings which sometimes surround the 
nucleus'? None of these questions have ever yet been 
resolved, in a satisfactory manner; nor can they be, 
in the present state of our knowledge. 

The envelope of comets would seem, at first thought, 
to be nothing more than a mass of vapour, disengaged 
from the nucleus, by the action of the sun; and such 
has been the opinion of Newton, and others. But 
this explanation, simple as it is, renders no account of 
the formation of the concentrick rings, round the nu- 
cleus, sometimes visible and sometimes not; nor of the 
variable positions of the envelope and tail, relatively 
to the sun, nor of the augmentation and diminution of 
the volume of these, &c. 

Upon this last point, however, a theory has not been 
wanting. Heveleus advanced the opinion that the en- 
velope augments in diameter, as the comet recedes 
from the sun; and Newton explained this result by 
supposing that the tails of comets, being formed, as 
he thought, from their envelopes, these last should 
diminish, in volume, in proportion as they approached 
the sun; and, reciprocally, should augment in dimen- 
sions, after the passage of the perihelion, when the 
material of the tail is again returned to the mass of 
the envelope. Nevertheless, it appeared difficult to 
admit that a gaseous mass should dilate itself in pro- 
portion as it became elongated from the sun, and con- 
sequently was passing into a region of greater cold; 
and the important remark of Heveleus obtained but 
little favour, until the comet of a short period gave 
confirmation of the fact — whatever may be the cause. 

During the last appearance of Halley's comet, it 
was observed, at the Cape of Good Hope, by the 



OF COMETS. 201 

younger Herschel, after its perihelion passage. He 
says of it, then, that its physical aspect was entirely 
changed. For a long time it had no tail. The para- 
bolick envelope of the head was formed under his very 
eye, as it were, with so great rapidity that its visible 
volume, which was well defined, was more than dou- 
bled in the space of twenty-four hours, computing 
from the morning of the 26th of January, 1836! On 
this occasion, in repeating, after an interval of three 
hours, the micrometick observations of the well defi- 
ned parts that it had when he commenced, he found 
an augmentation of their linear dimensions, amounting 
to one sixteenth of their entire size! This extraordinary 
dilation continued until the paraboloid became so great 
and of so feeble a light as finally to disappear, entirely, 
leaving only the nucleus and the tail visible. Another, 
rind very singular particular was the existence of a 
very small interiour comet, having a head and tail 
complete; its nucleus being that of the general mass. 
This dilated lass rapidly than the envelope. Before 
observations ceased the tail itself became invisible. 
On the other hand, Valz, satisfied himself, after care- 
ful research and examination, that while some comets 
contract, when approaching the sun, others do not, 
but on the contrary, dilate; and that of this latter class 
is the comet of Halley. 

Kepler thought that the formation of the tails of 
comets was the result of the impulsion of the solar 
rays, which detached the lighter portions of the enve- 
lope, and forced them to a distance behind the comet. 
But to render this explanation admissible it is first ne- 
cessary to prove that the solar rays are endowed with 
an impulsive force; for the most delicate experiments 
have hitherto failed to render such force perceptible. 
This force shown and admitted, it will still remain to 
be demonstrated why the tail is not always situated 
opposite to the sun; whv there are sometimes several 
" 17* 



202 OF COMETS. 

tails, making, one with another, so great angles; why 
they form, and again vanish, in so short periods of 
time; why some of them have a rapid rotary motion; 
and finall)-, why ?ome comets, of which the envelope 
seems very light and delicate, exhibited no trace of 
this appendage. 

A crowd of other theories, more or less ingenious, 
have been proposed; but they ail equally fail to explain 
the phenomena. 

Are comets self luminous bodies, or do they reflect 
the borrowed light of the sun, like the planets'? This 
important question has not yet received a complete so- 
lution; but there exist several methods by which such 
solution may be obtained. If observation should detect 
the phenomena of phases, in comets, this would dissi- 
pate all uncertainty. In default of phases, the pheno- 
mena of the polarization of light may conduct to the 
desired result. A third method, of which the applica- 
tion would probably leave no doubt, is the following: 

Suppose a detached and isolated luminous point, 
without sensible dimensions, which sends forth, upon 
every side, into space, particles of light. If we receive, 
at the distance of a yard, for instance, these luminous 
particles upon the surface of a sphere, of one yard ra- 
dius, they will be uniformly diffused over this surface, 
If we receive them at the distance of 2, 3, or 100 yards, 
the spheres, of course will be of 2, 3, or 100 yards 
radius, and the luminous particles will here, also, be 
uniformly diffused over the surface upon which they 
are received, but they will be farther from each other, 
in proportion to the increase of the surfaces of the 
spheres. Now, geometry demonstrates that the sur- 
faces of spheres increase proportionally to the squares 
of their radii: the separating distances, then, between 
the luminous particles would also be proportional to 
the squares of the radii of the spheres; or, in other 
words, to the squares of the distances at which the 



OF COMETS. 203 

luminous particles are received upon the surface of 
the sphere. And, as the intensity of the light which 
enlightens an object is in the ratio of the number of 
luminous rays which fall upon it, we thus arrive at 
this law, namely, that the luminous intensity of a point 
diminishes 'proportionally to the squares of the distan- 
ces. 

We have supposed, in the above case, a luminous 
point, without sensible dimensions: let us now elarge 
this point, so as to give it an appreciable extent of 
surface. 

It is evident that each point of this enlightened sur- 
face, like the isolated point of which we have spoken, 
will send forth light which will be enfeebled in the in- 
verse ratio of the squares of the distances; yet the 
number of luminous points being augmented, the total 
quantity of light emitted will be greater: from whence 
flows this consequence, namely, that at equal distances 
the intensity of the light is in proportion to the number 
of enlightening points. 

We have, then, arrived at this double result, namely, 
that the enlightening property of a luminous surface is, 
on the one part, proportioned to its extent; and on the 
other, in the inverse ratio of the squares of the distan- 
ces. 

The consequence of this law is, that the intensity of 
a luminous surface should appear the same, at whate- 
ver distance it may be situated, provided that it always 
subtends a sensible angle. 

That this may not, at first, appear contradictory to 
the very law from which we have deduced it, we will 
observe that, in the second case, it is the intensity of a 
luminous surface that is spoken of, while in the first it 
is the enlightening property of that surface. 

When we wish to compare, not the enlightening 
property, but the luminous intensity of two surfaces, 
it is necessary to take equal portions of each, and see 



£04 OP COMETS. 

which is the most brilliant. This supposed, we say 
that, two luminous surfaces being given, if portions of 
these be viewed, by the eye, through equal openings, 
thus exposing to sight portions of the same dimensions, 
and these portions appear to have the same intensity, 
the same will still be true, to whatever distance one of 
these surfaces may be removed, 'provided that the open- 
ing through which the part is seen shall always appear 
full 

In short; if, on the one hand, each luminous point 
sends to the eye a number of raj^s which is in the in- 
Terse ratio of the squares of the distances; on the other, 
the number of luminous points which the eye discovers, 
through the same opening, increases in the same pro- 
portion. The intensity of the visible portion of a lu- 
minous surface, then, under these circumstances, would 
not change. The sun, for instance, seen fom Uranus, 
would appear a circle of one hundred seconds: now, 
take of the surface of the sun a circle of one hundred 
seconds, by means of a screen, having a circular hole 
of this size through it, and we shall have, in size, and 
in light, the sun of Uranus. 

Let us now see what use can be made of these prin- 
ciples, in a solution of the question which we have in 
view, namely, are comets self luminous, or are they 
not? 

Or, the question may be stated thus: in what man- 
ner doss a comet cease to be visible to us? If its disap- 
pearance is an effect of the excessive diminution of its 
dimensions, and not of the enfeebling of its light, then 
the comet is self luminous; but if the comet, while it 
still has great visible dimensions, gradually grows dim, 
until finally its light is extinguished, such light was 
doubtless borrowed from the sun, and was only refec- 
ted by the comet. 

All observations, to the present period, appear to 
prove that this last cause of the disappearance is the 



OP COMETS. 205 

true one, and consequently that comets shine only by 
reflected light 

This consequence may not, however, be always cer- 
tain. It is now pretty clearly proved, as we have 
shown above, that the envelope of comets dilates in 
proportion as the comet's distance from the sun increa- 
ses. May it not be, then, that this progressive dila- 
tion would produce a gradual diminution of the comet's 
light? To clear this question, then, of all uncertainty, 
it will be necessary, hereafter, to consider this cause 
of the diminution of comets' light, and to demonstrate 
that it is insufficient to explain the disappearance of 
these bodies. This complication of the problem will 
not offer great difficulties. 

There are some other questions connected with co- 
metary astronomy, which we now proceed, succes- 
sively, to examine. 

Have comets a sensible influence upon the course of the 
seasons? 

To this question popular prejudices and superstitions 
have already abundantly responded in the affirmative; 
but a few words will suffice to dissipate this errour. 
We will first speak of facts, and let theoretical consi- 
derations follow. 

Astronomers have sought, by consulting the ther- 
mometrical observations which are regularly made, 
several times each day, at observatories and elsewhere, 
to determine whether those years which are fruitful in 
comets have a more elevated temperature than others: 
the result is, that no sensible difference has been dis- 
covered. 

This result is precisely that which theoretical data 
cause us to expect. By what species of agency is it 
supposed, indeed, that comets can modify our tempe- 
rature 1 ? These bodies cannot act, at a distance, upon 



206 OP COMETS. 

the earth except by attraction, by the emission of lu- 
minous or calorifick rays to it, and by the gaseous 
matter of their tails, which may mingle with our at- 
mosphere. 

The attractive force of comets, had it sufficient in- 
tensity, would produce tides, analogous to those caused 
by the attraction of the moon; but no one can see how 
this could result in an elevation of temperature. 

The luminous and calorifick rays which comets emit 
or reflect to the earth are certainly inadequate to any 
such result, since they have much less intensity than 
those which we receive from the moon; and which, 
concentrated in the focus of the largest lenses, produce 
no sensible effect, as already shown, when treating ot 
that satellite. 

Finally, the introduction, into the terrestrial atmos- 
phere, of a portion of the tails of comets cannot be 
assigned as a cause of the elevation of temperature 
which has been erroneously attributed to these bodies, 
because the tail of the comet of 1811, for example, 
(which comet was supposed by so many to have cau- 
sed the abundant crops that followed its appearance,) 
although it was one hundred millions of miles in length, 
yet the earth was at all times more than one hundred 
and fourteen millions of miles distant from the comet 
and of course far removed from any influence which 
its tail might be supposed to have exercised upon our 
atmosphere, had it been mingled with it. 

Is it possible for a comet to strike the earth, or any of 
the other planets? 

Comets move in all directions through space, descri- 
bing ellipses extremely elongated, which traverse our 
solar system, passing within the orbits of the planets, 
and consequently, often crossing them. It is not, then, 
an impossibility, that any one of the planets may en- 



OP COMETS. 207 

counter one of these bodies, in its course; and the stri- 
king of the earth, therefore, by a comet, is, strictly 
speaking, possible: but it is, at the same time, exceed- 
ingly improbable. 

The evidence in support of this proposition will be 
complete if we compare the small volume of the earth 
and planets with the immensity of the space in which 
these bodies move. ^The mathematical doctrine of 
probabilities furnishes the means of estimating, nume- 
rically, the chances of such a collision; and shows 
that such chance is as 1 to 280,000,000: that is to say,, 
upon the appearance of an unknown comet, there are 
280,000,000 of chances against 1 that it will not strike 
the earth. From this it is seen how ridiculous it would 
be for any man, during the few years that he has to 
pass upon the earth, to occupy himself with dreads and 
forebodings of such a danger. 

On the other hand, the effects of this shock might be 
frightful. If the earth were so struck as that its move- 
ment of translation, in its orbit, were destroyed, all 
bodies and substances which do not adhere firmly to 
the earth's surface, as animals, the waters, &c. would 
leave their places and be carried forward with a velo- 
city of about seven leagues per second. If the shock 
were encountered from a different direction, it would 
be easy to paint the effects of such a catastrophe. "The 
axis and the rotary motion of the earth would be chan- 
ged: the seas would abandon their ancient beds, to 
precipitate themselves towards the new equator: a 
great part of both men and animals would be either 
drowned in this universal deluge, or destroyed by the 
violent concussion thus produced: entire species would 
probably be destroyed; and all the monuments of hu- 
man industry be overturned. Such would be the disas- 
ters that the shock of a comet might produce, if its mas& 
bore any near proportion to that of the earth."* This, 

" Laplace. 



208 OP COMETS. 

however, from all that is known of comets, is not to 
be expected; the masses of the observed bodies, of this 
class, being far too insignificant to cause phenomena 
like those described. 

Has our globe ever encountered a comet, as the author 
we have just quoted supposed? 

There have been prominent men who have supposed 
that the axis of rotation of the earth was not always 
what it now is. These have supported their opinion 
upon arguments drawn from the fact that the different 
degrees measured upon each meridian between the 
pole and the equator, compared with each other, do 
not give the same value for the flattening at the poles. 
They have seen, in the difference of these results, the 
proof that the earth, at the time when, being still liquid, 
it assumed its spherical form, did not rotate upon the 
same axis that it does at present. 

But it is easy to see that a change of the axis could 
not be the cause of the discordances presented by the 
degrees furnished by observation, with those which 
result from a certain hypothesis of the flattening; since 
this disagreement does not observe any gradual and 
regular progress, but is wholly capricious, and without 
law. It is, undoubtedly, the result of local attractions, 
or geological contingencies, which are now known to 
exist as well in plains as in the vicinity of mountains. 

But we pass to other considerations. 

If a movement of rotation be impressed upon a sphe- 
rical, homogeneous body, freely suspended in space, 
its axis of rotation will remain perpetually invariable. 
If this body has any other form its axis of rotation may 
change every instant; and this multitude of axes, 
around each of which it will execute only part of its 
revolution, are called instantaneous axes of rotation. 
Geometry demonstrates that all bodies, whatever may 



OF COMETS. 209 

be their figure, and the variations of density, in their 
different parts, will turn constantly and invariably 
around three axes, perpendicular to each other, and 
passing through their centre of gravity. These are 
called the principal axes of rotation. 

This assumed, for it would be out of place to demon- 
strate it hove, the question arises, namely, is' the axis, 
around which the earth rotates, an instantaneous or a 
principal axis? In the first case, the axis changes at 
each instant, and the equator would be subject, in that 
event, to corresponding displacements. The terrestrial 
latitudes, which are only distances measured from the 
equator, would vary equally with the equator itself. 
Now, observations upon terrestrial latitude, which are 
made with extreme exactness, show no change of this 
kind — these latitudes being found constantly invaria- 
ble: it is therefore proved that the earth turns round a 
principal axis. 

It is easy to draw from this the proof that a comet 
has never struck the earth; since the effect of such a 
collision must have been to substitute an instantaneous 
axis for the principal axis around which it is proved 
that the earth rotates: in which case the terrestrial lati- 
tudes would now be subject to continual variations — • 
which, as we have seen, is not the case. Indeed it is 
a bare mathematical possibility that the effect of a 
shock of this kind might be to change the axis from an 
instantaneous to a principal one; but this case is so 
highly improbable that it can scarcely be said to call 
in question the proof we have given. 

What we have said above is based upon the suppo- 
sition that the earth is an entirely solid body. But its 
centre may be still liquid; as, indeed, is pretty gene- 
rally believed, at the present day; although some emi- 
nent men refuse their assent to this proposition. As- 
suming this condition of the earth to be true, the ques- 
tion then arises, can we, in such a case, deduce, with 
18 



210 OF COMETS. 

the same certainty, from the constancy of terrestrial 
latitudes, the consequence, namely, that the earth has 
never been struck by a comet? 

We think not; for, after the shock, of which the 
immediate effect would have been to precipitate, with 
great violence, towards the new equator, a portion of 
the interiour, liquid mass, which would not be able to 
assume a position there, without rupturing the solid 
crust of the earth, the continual displacement of the 
instantaneous axis causing an incessant transforma- 
tion of the fluid mass, it is possible that the result of 
the perpetual friction of this liquid, upon the solid 
shell, might produce a gradual diminution in the length 
of the curve described by the extremities of the instan- 
taneous axis, and, consequently, result, at last, in a 
movement of rotation round a principal axis. 

Can the earth pass into the tail of a ccmet? and what 
would be the consequences of such an event to its 
inhabitants? 

Comets have, in general, very little density; and 
they therefore can only attract very feebly the matter 
of which their tails are composed, because, as we have 
seen, all bodies exercise the force of attraction in pro- 
portion to their masses. 

Now, without any difficulty, we can conceive that 
the earth, of which the mass is ordinarily much greater 
than that of comets, might attract to itself, and miDgle 
with its atmosphere, some portion of the tails of such 
of these bodies as were favourably situated for such a 
result; particularly when we recollect at what an im- 
mense distance the extremities of these appendages 
sometimes are from the head of the comets to which 
they belong. 

As to the consequences of the introduction, into our 
atmosphere, of a new, gaseous clement, they must ne- 



OP COMETS. 211 

cessarily depend upon the nature and quantity of the 
matter: it might, therefore, be destructive of animal 
life, in whole or in part. But science has not yet re- 
gistered any event of this kind; and the connexion 
which many men have sought to establish between the 
appearance of comets and any of the moral or physi- ^ 

cal revolutions of the world, has not the slightest jr 
foundation in truth, but is the (legitimate offspring of f 
credulity and superstition. « 



Were the dry fogs of 1783 and of 1831 caused by por- 
tions of matter detached from the tails of comets? 

The fog of 1783 continued one month. It com- 
menced very nearly upon the same day at places 
greatly distant from each other. It extended from the 
north of Africa to Sweden. It occupied, also, a great 
part of North America; but it did not extend over the 
sea. In its extent upward, it reached above the tops 
of the highest mountains. It did not appear to be 
borne along, over the land, by winds; nor did the most 
abundant rains, or the strongest winds tend to its dis- 
persion. It exhaled a disagreeable odour, was very 
dry, as shown by the hygroscope, and was possessed 
of phosphorescent properties. 

Such were the facts; and there have been attempts 
made to explain them by supposing that this fog was 
the tail of a comet. But if this were so, why is it 
that no discovery was ever made of the head of the 
comet to which this tail belonged; for the fog was not 
such as to obscure the stars? This objection is funda- 
mental, and destroys, entirely, the proposed hypo- 
thesis. 

The same explanation is still less applicable to the 
fog of 1831, which offered so great a resemblance to 
that of 1783; for this fog having covered only a part 
of Europe, the invisibility of the comet from which the 



212 OF COMETB. 

tail proceeded would be still more surprising. Besides, 
all the points of the globe comprised between the same 
parallels of latitude would have been successively co- 
vered with this fog, by the effect of the rotation of the 
earth upon its axis: and yet this fog terminated at fifty 
leagues from the coast. 

ouch more Explanation of the origin 

of these extraordina: : found in the inte- 

riour revolutions 1 our globe is so often agita- 

ted. In 17S3. the same year of the fi: : fogs 

which we are considering, Calabria was overturned by 
those terrible earthquake ved more than 

of the inhabitants; Mount Hecla, in Iceland, 
sent forth one of the most stupendous eruption^ 
known: while r.i m the bottom of 

ire. 

old it, then, be a difficult thing to suppose that 

is : .atter. of an unknown natu: 
from the interiour of the ear - torn and 

agitated by these violent coinm: certainly 

would not; and the supposition th: the expla- 

nation is strongly supported by the remarkable circum- 
stance that, over extensive sea *t pre- 
vail. But we mean no more, here, than barely to in- 
dicate one of the hy pot!. id of whi 
would be possible to explain the origin of these dry 
fogs, without recurring to the immersion of the earth 
in the tail of a comet. 

upon the frica, 

something very similar to the phenomena that we have 
described. This is a dry, periodical fog, brought on, 
or at least occurring at the same time with, a 
known as the Ht :s furniture, in 

dwellings, to crack: warps the covers of bocks, as if 
they had been held against a fire: dries up grass and 
larger plants; and exercises, upon the human sys 
an influence no less unpleasant. This fog never ex- 



OP COMETS. 213 

tends much over the sea. Its producing cause is un- 
known. 

Has the moon ever been struck by a comet? 

We have seen that this satellite turns upon its axis 
in a time precisely equal to that which it employs to 
accomplish its revolution round the earth. The equal 
duration of the periods of these different movements 
has been thought to be explained by supposing that, 
at the time the moon, then in a fluid state, was assu- 
ming the form which corresponds to its movement of 
rotation, the attraction of the earth drew the moon into 
a lengthened form, its major axis being directed towards 
the centre of the earth. 

Now, if a comet had ever struck the moon, the 
shock would have destroyed the harmony which exists 
between the movements of rotation and of revolution of 
this planet, and consequently have caused the major 
axis of the moon to deviate from a right line to the 
centre of the earth. In such case, that major axis 
would execute a series of oscillatory movements, like 
those of a pendulum, round our earth: but, as nothing 
of this kind exists, it is considered evidence that a col- 
lision between a comet and the moon has never taken 
place. 

Has the moon been formerly a comet? 

The Arcadians, according to Lucian and Ovid, be- 
lieved themselves more ancient than the moon. Their 
ancestors, said they, inhabited the earth before the 
moon existed. This singular tradition has caused the 
inquiry to be raised whether the moon was not an- 
ciently a comet, which, passing near the earth, was 
so attracted by that body as to cause it to become its 
satellite. 

18* 



214 OF COMETS. 

There certainly is nothing impossible in this; but 
the considerations which have been offeredin'support 
of this opinion, have not the least value. As, upon 
this hypothesis, the moon, while yet a comet, must 
have had a very short perihelion distance, the attempt 
has been made to show, by the burned aspect of some 
of its mountains, the traces of that enormous heat 
which it has been supposed it must have experienced, 
while passing so near the sun. There is a confusion 
of language in this. It is very true that the appear- 
ances of ancient volcanick convulsions give to some 
points of the surface of the moon a burned appearance; 
but no appearance there, at the present day, can give 
any indication of the temperature which the moon may 
have formerly experienc 

Finally, the partisans of the above opinion will have 
to explain why the moon has no sensible atmosphere, 
while all the comets that have been seen have a gase- 
ous envelope surrounding them. If the moon was an- 
ciently a comet, what disposition has been made of its 
envelope? 

Is it possible that the earth shall become the satellite of 
a comet: and if so, what fate, in such an event, would 
await its inhabitants? 

That a comet should possess itself of our earth, so 
as to convert it into a satellite, it is only necessary 
that the comet have a mass sufficiently considerable, 
and that it pass sufficiently near the earth. Then it 
would, without doubt, snatch the earth from the attrac- 
tion of the sun, and bear it along with itself, as a sa- 
tellite, in its revolution round that luminary. But the 
great mass which it is necessary to allow to the comet, 
and the small distance which can exist between the 
earth and the comet, in fulfilling these conditions, ren- 
der this event very slightly probable. 



OF COMETS. 215 

Still, as such an event, speaking rigorously, is pos- 
sible, let us examine what, in this hypothesis, would 
be the fate of the inhabitants of the earth. Would 
our globe, as has often been asserted, experience the 
extremes of temperature? Would it successively be 
vitrified, volatilized and congealed? Would it become 
uninhabitable, and all the animal and vegetable species 
upon it be destroyed? 

Let us suppose, in replying to these questions, that 
the earth should become the satellite of a comet whose 
perihelion is very near the sun, and whose aphelion 
is very distant from it; as the comet of 1680, for 
example. 

This comet completes its revolution in 575 years, 
describing an ellipse of which the major axis is 138 
times greater than the mean distance of the earth from 
the sun. Its perihelion distance is extremely short. 
Newton calculated that, at its passage through its peri- 
helion, on the 8th of December, 1680, it experienced 
a heat 28,000 times greater than that which the earth 
receives in summer: and which he estimated at 2000 
times greater than that of red hot iron. 

But this conclusion is by no means admissible. For 
the solution of the problem which Newton proposed, 
it is necessary to know the condition of the surface 
and of the atmosphere of the comet of 1680. Nor is 
this all: for, suppose our globe placed in the position 
of that comet, at the time in question, and the prob- 
lem would not yet be solved. No doubt the earth, at 
first, would experience a temperature 28,000 times 
greater than that of summer; but very soon all the 
liquids which cover its surface would be converted into 
vapour, producing thick layers of clouds, which would 
weaken the action of the sun in a proportion that it 
is impossible to fix, numerically. 

Would it be less difficult to determine the tempera- 
ture of our globe, when it should have accompanied 



216 OP COMETS. 

the comet to its aphelion? In considering only the 
circumstance of distance, the earth should then be 
19,000 times less heated than it now is, in summer; 
that is to say, as it could not there receive from the 
sun any appreciable heat, it could only possess that 
portion of what it imbibed while at its perihelion 
which had not yet been dissipated: and if it had lost 
all this heat, then it would be of the same temperature 
as the surrounding space, which, according to the in- 
genious investigations of Fourier, cannot descend far- 
ther than 50° centigrade, equal to 58° of Fahrenheit, 
below zero. 

Now, experiments have proved that man can sup- 
port a degree of cold of from 49° to 50° centrigrade, 
below zero, and a heat of 130° above, when he is 
placed in certain hygrometrical conditions. There is 
no proof, then, that the human species would be anni- 
hilated, by thermometrical influences, if the earth 
were to become the satellite of a comet; and even of 
a comet of as great eccentricity of orbit and length 
of period as that of 1680. 

These considerations upon the limits between which 
the temperatures of the celestial globes may oscillate, 
are of a nature to render their inhabitation less prob- 
lematical, in the eyes of such persons as find it diffi- 
cult to conceive of the existence of beings formed 
upon a system of organization totally different from 
our own. 

Was the deluge occasioned by a comet? 

There can exist no doubt, at this day, that our 
globe has been subjected, in past ages, to great and 
extensive changes, some of which have evidently been 
accompanied, if not accomplished, by the violence of 
powerful agents: nor is there room to doubt that the 
oceans have changed their beds, so that continents 
have been alternately submerged and laid bare. To 



OP COMETS. 217 

explain these phenomena, theorists have sought the 
agency of comets. Let us examine these explana- 
tions. 

Whiston proposed one which he adapted to all the 
circumstances of Noah's deluge, as recorded in the 
book of Genesis. He supposed — and we should here 
remark that this supposition has nothing inadmissible 
— that the comet of 16S0 was in the vicinity of 
the earth at the time of this deluge. He farthermore 
supposed the earth to have anciently been a comet, 
and he believed it composed of a solid nucleus, sur- 
rounded by two concentrick orbs, the interiour one 
constituted of a heavy fluid, and the outer one of 
water: and still outside of this water he placed the 
solid crust of the earth, upon which we tread. 

This assumed, he placed, at the epoch of the deluge, 
the comet of_16S0 only some three or four thousand 
leagues from the earth. This body, exercising, by 
reason of its extreme proximity, a powerful attraction 
upon the interiour liquids of the earth, produced an 
immense tide, which burst the solid crust composing 
the earth's surface, and precipitated the liquid mass 
upon the continents. Such is Whiston's interpretation 
of the breaking up of the fountains of the great 
deep. 

As to the opening of the icindoics of Heaven, as 
ton could not discover such a result in forty days 
of ordinary rain, which could produce only slight re- 
he sought this effect in the tail of his comet, 
which he supposed to have spread through the atmo- 
sphere of our globe a sufficient quantity of aqueous 
vapour to produce the amount of rain spoken of. 

This fanciful theory long enjoyed great celebrity, 
in England, where its author was Professor of Mathe- 
maticks, at Cambridge college; but it is almost needless 
to say that it is now wholly exploded, being unable to 
sustain a moment's examination. 



218 OP COMETS. 

We shall not speak of the physical constitution 
which Whiston gave to the earth, and which geology, 
at this day, in no wise adopts or confirms. We limit 
ourselves to the remark that his gratuitous suppositions 
upon the proximity and the mass of the comet of 1680 
are insufficient to an explanation of the phenomena. 

For instance, the motion of this comet being ex- 
tremely rapid, its attraction was not exercised suffi- 
ciently long upon any one point of the earth to pro- 
duce the immense tide of which we have spoken. 

Farthermore, this famous comet passed very near 
the earth on the 21st of November, 1680; and it is 
demonstrated that, at the epoch of the deluge, its dis- 
tance was not less. Yet upon that occasion, namely, 
1680, it neither broke up the fountains of the great 
deep, nor opened the windows of Heaven. Whiston's 
explanations, then, are inadmissible. 

Halley, who embraced this view, though in a more 
general manner, has endeavoured to explain the pre- 
sence of marine productions far from any ocean, and 
even upon the tops of the highest mountains, by sup- 
posing the earth to have been struck by a comet. 

We have already examined the question, namely, 
whether the earth has ever been struck by a comet, 
and shown that it probably has not. We will add, 
here, that, supposing, for a moment, such to have 
been the fact, still we seek, in vain, in the effects of 
such a concussion, any satisfactory explanation of the 
observed phenomena. The stratification of the marine 
deposits; the extent and regularity of these forma- 
tions; their positions; the perfect preservation of the 
most delicate and fragile shells; and we ma}- add, 
vegetable leaves and flowers, all exclude the idea of a 
violent transport of these materials, and demonstrate 
that they were quietly deposited where we find them. 

The explanation of these phenomena no longer pre- 
sents any difficulty, since science has been enriched 



OF COMETS. 219 

by the accurate and extended views of Mons. Elie de 
Beaumont upon the formation of mountains by the 
agency of elevating power from beneath; but which, 
as they belong not to our subject, but to geology, can- 
not be detailed here. 

Have the different points of our globe suddenly changed 
their latitude, in consequence of a collision between 
the earth and a comet? 

We find, in the different climates of Europe, bones 
of the rhinoceros, the elephant, and other animals, 
which are not able to live, at the present day, in the 
latitudes where many of these bones are found. We 
must, then, suppose, either that the climate of Europe 
has been considerably lowered, in its temperature, or 
that, in one of those violent commotions, of which 
our globe offers some traces, these bones have been 
transported to higher latitudes, by currents flowing in 
a northerly direction. 

But neither of these hypotheses is in any way 
adapted to the explanation of the two modern discove- 
ries which have so much occupied the attention of the 
learned. There was found, in 1771, upon the shore 
of the Wilhoui, in Siberia, and at several feet beneath 
the surface of the ground, a rhinoceros in a state of 
perfect preservation, neither the flesh nor the skin ap- 
pearing to have undergone any change. Some years 
since that event, namely, in 1799, there was found, 
near the mouth of the Lena, upon the shores of the 
Frozen Ocean, an enormous elephant, enclosed in a 
mass of frozen mud, and so well preserved that the 
dogs readily ate its flesh. 

How can we explain the presence of these two un- 
wieldly animals in regions so distant from those where 
we now find them living? Here the agency of the 
supposed currents is inadmissible, for if these animals 



220 OP COMETS. 

had not been frozen immediately after death, the pro- 
cess of putrefaction would have destroyed them. 
They have, then, lived in the place where they were 
found. Then, on the one hand, it has been supposed 
that Siberia has formerly enjoyed a more elevated 
temperature than at present, because elephants and 
rhinoceroses have inhabited there; and, on the other, 
that the catastrophe in which these animals perished 
has suddenly rendered this a region of extreme cold. 

Now, from these deductions, to the shock of a comet, 
as their producing cause, is but a single step; for we 
know no other cause than this which would be capable 
of producing a sudden change in the latitudes of our 
globe, accompanied by extreme violence. 

The question, then, occurs, is this explanation ad- 
missible'? We think not. 

And, first, is it established that the elephant of Lena, 
and the rhinoceros of Wilhoui may not have lived 
under the present climate of Siberia? There is much 
room to doubt that this has been done: for these ani- 
mals, otherwise similar, in form and magnitude, to 
those which at present inhabit Africa and Asia, were 
distinguished by one circumstance which is particularly 
worthy of observation, namely, they were covered 
with a species of fur. The skin of the rhinoceros 
was thickly studded with bristles, or a species of 
coarse hair, three or four inches in length; and that 
of the elephant was covered with a long, black hair, 
mixed with a thick set coat of reddish wool, while the 
neck of the animal was furnished with a long mane. 
These peculiarities are all remarkable, and tend di- 
rectly to the belief that these animals were fitted, by 
nature, to inhabit the cold regions of the north. 

In addition to this, that celebrated traveller, Hum- 
boldt, has recently ascertained that the royal tiger, an 
animal which we have ever considered as belonging 
only to the hottest countries, is now a resident of Asia, 



OF COMETS. 221 

in very high latitudes; and that it makes its way, in 
summer, to the western side of the Altayan mountains. 
Why, then, could not these elephants, with thick, 
woolly covering, make their way, daring summer, even 
to Siberia? Supposing them once there, the most 
ordinary accident, such as a fall, for instance, would 
be sufficient to envelop them in frozen layers, capable 
of preserving them from all putrefaction; for, in these 
latitudes, the entire body of the earth, at a depth of 
only some twelve or fifteen feet, remains perpetually 
frozen. 

It is not necessary, then, in order to explain the 
discoveries at Lena and YTilhoui, to recur to the 
agency of a comet, through its concussion with the 
earth. On the contrary, this supposition, which we 
have elsewhere considered inadmissible, can yield us 
no explanation here. Let us suppose, for instance, 
that Siberia was formerly near the equator: we must 
then also admit, that it was, in that position, covered 
some five leagues deep by water, owing to the rotary 
motion of the earth: and where, we may ask, were 
our rhinoceros and our elephant, then? Mons. Elie 
de Beaumont has ingeniously attempted to solve the 
problem involved by the presence of these animals 
where they were found, by connecting it with his 
theory of the formation of mountains. He supposes 
that the Tian-Chan was elevated in winter, in a coun- 
try of which the valleys were inhabited by these ani- 
mals, and the mountains of which being covered with 
snow, the hot rapours issuing from the earth, at the 
moment of the volcanick action which upheaved the 
part in question, dissolved a part of the snows upon 
the older mountains, producing a vast current of water, 
at a very low temperature. This current, by bearing 
along the dead bodies which occurred in its course, 
would carry them, in about eight days, and without 
exposing them to putrefaction, to the latitudes of Sibe- 
19 



222 OP COMETS. 

ria, where, of course, they would be at once thoroughly 
frozen. 

What has caused so widely extended a depression of 
land, over a great part of Asia? and is this depres- 
sion to he ascribed to the blow of a comet? 

There is, in Asia, a vast region of some 18,000 
square leagues, occupied, in great part, by the Cas- 
pian Sea, and where there are many populous towns, 
which is depressed about 330 feet below the level of 
the Black Sea, and of the Ocean. 

To explain this enormous depression of a great por- 
tion of an entire country, recourse has been had, as 
in so many and so various other cases, to the agency 
of a comet, which is assumed to have struck the earth 
in this depressed part. 

This explanation was proposed by Halley; but at 
this day it is wholly abandoned. The earth, as we 
have seen, has never been struck by a comet; and the 
geographical phenomenon in question is explicable 
without resort to this supposition. 

It is the generally received opinion of the present 
day that mountains are formed by the elevation of ma- 
terials from beneath the surface of the earth, being 
forced upward, often in a melted state, by volcanick 
agency. Now the necessary consequence of this 
must be to produce void spaces, beneath some or all 
portions of the surrounding surface; and certainly the 
possibility follows that such surfaces may subsequently 
sink into the cavities thus formed. 

By casting our eyes over the geographical chart we 
shall see that Asia is more remarkable, for elevated 
clusters, than any other part of the world, and that 
around the depressed region of which we have spoken, 
there rise a series of great mountain chains; the Iran, 
the Himaleh, the Kuen-Lun, the Thian-Chan, the Cau- 
casus; and the mountains of Armenia, those of Erze- 



OF COMETS. 223 

rum, &c. Now why may not the elevation of these 
vast masses have caused the depression of the inter- 
mediate surface of the globe? 

This explanation appears still more plausible, when 
we add that, in the regions in question, complete sta- 
bility is not yet arrived at, and that the bottom of the 
Caspian Sea, for instance, furnishes evidence of alte- 
ration, in elevation and depression. 

We have seen that comets, in various ages, and 
among different people, have been objects of the 
greatest alarm; but we should add that all persons 
have not yielded to these exciting fears, but have held 
rational views in regard to these bodies, at times when 
such views found no favour with the mass of man- 
kind. The period when these views were first enter- 
tained, and the people by whom they were originally 
promulgated, are alike unknown to us. They have, 
indeed, often been ascribed to Seneca; who said, of 
comets, that they are the enduring works of nature, 
and have their appointed route, in which to move; and 
although they elongate themselves, yet they cease not 
to exist. They are not confined to the zodiack; but 
the heavens are free in all their parts, and whereever 
there is space there may be motion, &c. He also pre- 
dicted that future ages would make farther discove- 
ries, upon their character, the nature of their orbits, 
and the periods of their return. Of this famous pre- 
diction of Seneca much has been written, claiming, 
for him, the honour of its first promulgation. But, in 
rendering to Seneca that justice which is due to him 
for the adoption of these philosophical views, we must 
not forget that he prophesied, as did the astrologers, 
namely, after the event. It was from the Chaldeans 
that Seneca derived his philosophy, upon this subject, 
and also his famous prediction of the future return of 
oomets; which fact is historically asserted to have 
been early known to that people. 



224 ECLIPSES 

CHAPTER TWELFTH. 

OF ECLIPSES. 

Eclipses, like comets, were of old an object of popu- 
lar alarm and affright; but at this day there is no one, 
among civilized nations, but knows that these pheno- 
mena are a consequence of the known laws of nature, 
and that they are predicted, by astronomers, with the 
same exactness as are the returns of day and night. 

ECLIPSES OP THE 3IOON. 

The earth being a round, opaque body, the sun can 
only enlighten one half of its surface at any one time; 
from which it follows that the earth must, at all times 
project a shadow upon the side opposite to the sun. 
Let us inquire what is the form of this shadow? and 
what are its dimensions? If the sun and the earth 
were of the same magnitude, the shadow would be 
cylindrical, and of infinite extent; but as the earth is 
much less than the sun, the shadow which it projects 
forms a cone sufficiently long to reach the moon, but 
not so long as to extend to I v <Jars. The length of this 
conical shadow has been computed at some 300,000 
leagues. Upon the sides of this cone are shadows 
less dark, formed by the interception of a part, only, 
of the rays of the sun; and of these shadows the in- 
tensity decreases in proportion to the increase of dis- 
tance from the conical shadow. This tint, interme- 
diate between light and perfect shadow, has received 
the name of penumbra. To determine the limits of 
this partial or imperfect shadow, two lines must be 
drawn, which, commencing, one at each border of the 
sun, and crossing before they reach the earth, each 
shall just touch the border of the earth, and be pro- 
longed beyond that planet. The prolongation of these 



ECLIPSES OF THE MOON, 



225 



-lines will bound, beyond the earth, a truncated cone, 
which is that formed by the penumbra. To illustrate 
Fig. 37. tm s, suppose, fig. 37, S the sun, and 
E the earth. The cone of the earth's 
shadow, a b f, terminates at f, the 
point where the rays emanating from 
the borders of the sun meet, after 
having grazed, as it were, the bor- 
ders of the earth; while the trunca- 
ted cone a b c d is that formed by 
the penumbra. This is cut off, or 
truncated, at its top, so that its width, 
there, is equal to the diameter of the 
earth, instead of being a mere point, 
; as in a perfect cone. 

It is now evident that whenever the 
earth, in its motion round the sun, 
occupies a position on a right line, or 
nearly so, between the sun and the 
moon, this last body will of necessity 
be obscured, or eclipsed. This eclipse 
will be total or partial, according as 
the moon is wholly or partially in- 
cluded in the conical shadow, a bf, 
fig. 37, of the earth. The eclipse 
will be central, whenever the centre 
of the moon coincides exactly with 
that of the earth's shadow. 

If the plane in which the moon 
moves was not inclined to the eclip- 
°tick, this planet would be eclipsed 
-at every full moon; but as the orbit which it describes 
cuts the ecliptick, in the line of the nodes, it must as- 
sume different positions relatively to that plane. If, 
at opposition, the moon is distant from its nodes, it 
will pass upon one side or the other of the earth's 
shadow, without falling into it; which happens in moat 
19* 



226 ECLIPSES OF THE MOON. 

cases of opposition: but if the line which joins the 
centres of the sun, earth and moon, be a right line, or 
nearly so, which takes place when this last body is in 
the nodes, or near that position, there must, of neces- 
sity, ensue an eclipse. 

For the purpose, of designating the extent of an 
eclipse, astronomers suppose the moon divided into 
twelve equal and parallel zones, called digits. Then, 
when there is one third or one half of the moon 
eclipsed, we say that it is eclipsed four, or six digits. 
If the eclipse is total, and the diameter of the shadow 
is greater than that of the moon, we say that the 
eclipse is more than twelve digits, and the number of 
these digits is determined accordingly. 

In eclipses of the moon, the place of the spectator 
makes no difference as to the phenomenon, which 
must happen at the same instant of absolute time, to 
all observers, in whatever part of space they are 
situated; and if the whole eclipse be visible, at all, 
that is, be above the horizon, it must everywhere have 
the same magnitude. In the case of the moon it is 
always upon the eastern edge or side of the moon that 
eclipses of that body commence. 

The moon, in approaching the conical shadow of 
the earth, loses its light by slow and imperceptible de- 
grees, by reason of its entrance into the penumbra, of 
which we have seen that the intensity augments gra- 
dually, from its outer border to the very time of con- 
tact with the earth's shadow. When buried in the 
earth's shadow, the moon does not usually entirely 
disappear, even when the eclipse is total, because 
refraction throws some rays of light upon it, within 
the cone of the earth's shadow. Yet instances have 
occurred when the disappearance of the moon was 
complete, owing to clouds in the atmosphere, which 
interfered with the refraction in question. 



ECLIPSES OF THE SUN. *2*27 

We have already said that the eclipses of the moon 
are visible from all points of the earth which have 
the moon above the horizon, and that such eclipses 
have, for all these points, the same extent; but we 
should add that the hour of the day at which such 
eclipse will be viewed must vary with the longitude of 
the spectator, and this circumstance furnishes the 
means of determining the longitude of those places 
when accurate observations are made. Eclipses of 
the moon never exceed four hours, and most of them 
have a much less duration. 

ECLIPSES OF THE SUX. 

When the moon is interposed between the sun and 
the earth, the sun is eclipsed. The eclipse is partial, 
when the moon only hides a part of the disk of the 
sun; it is total when the entire disk of the sun is 
hidden; and it is annular when the sun, covered by 
the moon, still shows the border of his disk, for a 
greater or less width, entirely round the moon, con- 
stituting a ring of light outside that body. Lastly, 
the eclipse is central when the observer is situated 
upon the prolongation of the line which joins the 
centres of the sun and moon. 

The moon being nearly of the same figure as the 
earth, its shadow and penumbra are found in the same 
manner; but, as the moon is much less than the earth, 
the cone of its shadow is never of sufficient magnitude 
to cover the entire surface of the earth. Eclipses of 
the sun never take place at the same time throughout 
the earth; and it is easy to see that an eclipse of this 
body which shall be total at one place, on the earth, 
max not be visible at another, although the observer 
at this last position, should have the sun above the 
horizon. But, as the moon passes before all the points 
of the solar disk, it hides this, successively, from 



228 KCLISPES OP THE SUN. 

different parts of the earth, in the order of its move- 
ment, from west to east. In most solar eclipses the 
disk of the moon is covered by a pale light which is 
reflected to it by the enlightened portion of the earth's 
surface. 

The apparent diameter of the moon, when at its 
maximum, exceeds the minimum diameter of the sun 
by only 1' 38". So that the longest duration of a 
total, solar eclipse, cannot exceed the time which the 
moon requires to move over 1' 38" of a degree, which 
is about 7 m 58 s of time. 

Solar eclipses, like lunar ones, are estimated in 
digits, by supposing the sun divided into twelve equal 
parts, also called digits. 

We will illustrate, by a diagram, the general phe- 
nomena of these eclipses. 

Figure 38, suppose S the sun, Y Y the earth, M 
the moon, and AMP the moon's orbit. If we draw 
the lines Wee and V d e, the dark space, c d e, com- 
prised between them will be the .conical shadow of the 
moon: the lines W d h and V c g mark the limits of 
the penumbra, T T. This assumed, conceive the 
moon moving in her orbit, from west .to east, as from 
M to P. Now, an observer placed at I would see the 
eastern limb of the moon, d, seem to touch the west- 
ern limb of the sun, W, at the commencement of the 
eclipse, to him. But, at the same instant the western 
border of the moon, c, quits the eastern limb of the 
sun, at V, which is the end of the eclipse to an obser- 
ver placed at a. It is now evident that, in this case, 
the sun has been eclipsed at all the points between a 
and b. But it is also evident, according to the figure, 
that the sun was not totally eclipsed except for a very 
small part of the earth at a time, because it is only 
the extremity of the shadow which reaches the earth. 
In this diagram the diameter of the sun is seen divi- 
ided into twelve equal parts, or digits. 



ECLIPSES OF THE SUN. 



229 



The return or repetition of eclipses does not take 

place, except at considerable intervals of time. They 

Fig.38. 




M TC US «0 l- 69 



can only happen at the syzygies. The synodical 
revolution of the nodes embraces the period of 
346 d 14 h 52 ra 16 s ; being, to the synodical revolution of 
the moon, very nearly in the proportion of 223 to 19. 



230 ECLIPSES OP THE SUN. 

After a period of 223 lunations, then, the sun and 
moon return to the same position, relatively to the 
lunar node; and the series of eclipses returns nearly 
in the same order. The recurrence of eclipses is de- 
termined by an observance of this period; and calcu- 
lation has demonstrated that such return takes place 
about every 18 £ years. This period is thought to be 
the Saros of the Chaldean astronomers; and their pre- 
dictions of eclipses were probably founded upon it. 

As total eclipses of the sun are very rare, some no- 
tices of the one which was visible in the United States, 
on the 16th of June, 1806, may not be uninteresting. 

The late distinguished mathematician, Dr. Bowditch, 
of Boston, observed this eclipse, at Salem, Massachu- 
setts. He says of it: "On the day of the eclipse the 
weather was remarkably fine, scarcely a cloud being 
visible in any part of the heavens. Four or five mi- 
nutes before the commencement of the eclipse I began 
to observe that part of the sun's limb, where the first 
contact was expected to take place, and at 10 h 8 m 28 s 
by the chronometer, I observed tha first impression on 
the limb. In two seconds the indentation of the limb 
was quite perceptible. As the eclipse advanced there 
did not appear to be so great a diminution of the light 
as was generally expected, and it was not till the sun 
was nearly covered that the darkness was very sen- 
sible. A few minutes before this time I recommenced 
my observations with the telescope, and at ll h 37 m 30 s , 
by the chronometer, the sun's surface was wholly co- 
vered. The last ray of light from the sun's limb 
disappeared so instantaneously, that it seemed as if 
there could not have been a mistake of a second in 
this phase. The whole of the moon was then seen, 
surrounded by a luminous appearance of considerable 
extent, such as has generally been taken notice of, in 
total eclipses of the sun. This light, with a crepus- 
cular brightness round the horizon, prevented the dark- 



ECLIPSES OP THE SUN. 231 

ness from being great during the time that the sun's 
surface was wholly covered. The degree of light can 
be estimated by the number of stars visible to the 
naked eye. Those I took notice of were Capella, 
Aldebaran, Sirius, Procyon, the three bright stars in 
the bell? of Orion, and the star a in his shoulder. 
Venus and Mars were also visible. 

"As the time drew near for observing the end of the 
total darkness, I took notice that there was a visible in- 
crease of light, in the atmosphere, for about two 
seconds before any part of the sun's limb was visible 
in the telescope; but at ll h 32 m 18 s , by the chronome- 
ter, the light burst forth, with great splendour. After 
this the light appeared to increase much faster than it 
had decreased, and in a short time it was as light as 
in a common cloudy day." The whole duration of 
this eclipse, at Salem, after correcting the chronome- 
ters' errour, was 2 h 44 m 18 s ; and the duration of total 
darkness, 4 m 48 s . 

At Pawlet, Vermont, "the light of the sun instantly 
ceased for a moment or second of time, and as instan- 
taneously reappeared." 

At Tarpaulin Cave, on the coast of Massachusetts, 
this eclipse was total; while at five miles W. S. W. 
of this place "the eclipse was not total, but the lucid 
part of the sun was very small, and of a circular form, 
and appeared very much agitated; suddenly changing 
its place round the south limb of the moon.' 7 

An observer, at another station, describes the eclipse 
as "not total, but the sun's limb appeared to be redu- 
ced to a small, circular thread, or, rather, like a very 
fine horn, the upper end of which broke into a drop and 
instantly disappeared. The lucid part of the sun evi- 
dently decreased until ll h 16 m 52 s . The position of 
the lucid part, at this time seemed to change suddenly 
to the east, and in a few minutes appeared on that part 
of the sun where the eclipse began, and the light in- 



232' ECLIPSES OF THE SUN. 

stantly increased before there was any appearance, to 
the eye, of an increase of the lucid part." 

Such are among the phenomena which scientifick 
men have recorded of this rare and imposing occur- 
rence. 

The author of this observed the same eclipse, from 
a point near the northern line of Massachusetts. The 
day was without clouds, and of unusual brightness. 
The diminution of light, as the obscuration of the sun 
progressed, was not so rapid as had been anticipated; 
but the period of total obscuration was one indescriba- 
bly impressive. The sunless sky appeared, indeed, 
"the pall of a past world;'" 7 and the darkness, though 
not impenetrable, seemed the sombre precursor of na- 
ture's dissolution. A damp chill was every where per- 
ceptible, in the open air; and while all men gazed, in 
mute silence, upon the heavens, domestick fowls and 
the birds of the grove betook themselves to the perch, 
while the beasts of the field, suspending every effort, 
stood motionless with astonishment. 

To observations of ancient eclipses, chronology has, 
in some instances, been greatly indebted for the correc- 
tion, or the confirmation of its dates. Through the 
gloom of the palpable night of antiquity, some few 
rays, from the lights of astronomy, have penetrated; 
and certainty, amid surrounding chaos, is restored 
whereever astronomical observations can be recognised. 
Such facts as can be coupled with these, become fixed 
points, and serve as asylums for the secure repose of 
such inquirers as have been led astray by uncertainty, 
amid the darkness of antiquity. But these observa- 
tions are rare; and to superstition we must acknowl- 
edge ourselves indebted for such as we have. It is, per- 
haps, singular, that this long night of darkness should 
be illumined only by such traits as superstition has 
unintentionally preserved to us. These traits are the 
phenomena of eclipses, which the terrours of the 



ECLIPSES OP THE SUN. - 233 

people caused them to consecrate to posterity. When- 
ever we find, in ancient annals, details of occurrences 
without date or with a false one, but accompanied with 
a recital of an eclipse, astronomy, supported by a 
knowledge of the motions of the sun and moon, will 
immediately, by calculation, ascend into antiquity, and, 
by examining every eclipse there can have been, will 
finally identify the one that fell upon the specified day 
and hour — by which means the date of the event be- 
comes fixed. For the purpose of preserving such re- 
searches and calculations, for future practical applica- 
tion, two Benedictine Monks, of the last century, pre- 
pared a work which they entitled the Art of verifying 
Dates; or, of fixing the dates of historical events, by 
astronomy, through the medium of those eclipses which 
the ancients have preserved to us. 

Among the Chinese, where superstition, in the guise 
of astrology, is recognised by, and forms part of the 
administration of national affairs, all ancient chrono- 
logy is founded upon the observation of eclipses; and 
it is upon this undeniable evidence, this testimony 
which admits of no cavil, that, at the epoch of the em- 
perour Yao, more than two thousand years before our 
era, astronomy was cultivated in China. The calen- 
dar, and the announcement of eclipses were important 
objects, for which a tribunal of mathematicks had been 
created. They then observed the meridian shadows 
of the gnomon, at the solstices, and the passages of the 
stars through the meridian. They also determined 
the position of the moon, relatively to the stars, at the 
times of eclipses; and these gave the sidereal positions 
of the sun and of the solstices. 
20 



234 TIDES. 



CHAPTER THIRTEENTH. 

OP THE TIDES. 

The phenomena of the tides, which naturally next 
claim our attention, will receive their explanation in 
this chapter. 

A multitude of theories have, in times past, been 
presented, upon the cause of these regular and period- 
ick fluctuations of the ocean; and although their rela- 
tion with the movements of the moon was observed 
at a very high period of antiquity, yet it was Kepler 
who first discovered that it was the attraction exerci- 
sed by that planet which is the producing cause of 
tides. Newton afterwards showed that this opinion is 
in harmony with the laws of gravitation ; and reason- 
ing from the principles laid down by Kepler, he ex- 
plained how the tides rise upon two sides of the earth 
at the same time, one of them being opposite to the 
moon. This theory of the tides is received, at this 
day, without dispute, as their true solution. 

The waters of the ocean enjoy a degree of mobility y 
in their particles, which causes them to yield to the 
slightest impressions, while the ocean and all the great 
seas are open, and connected with each other: these 
circumstances contribute greatly to the production of 
the tides; which are chiefly caused by the combined 
action of the sun and moon. 

We will first consider the action of the moon. It is 
evident that it is the inequality of the action of this 
body which chiefly causes the tides; and that these 
would not exist if the moon acted in a uniform manner 
over the entire extent of the ocean; in which case the 
equilibrium of all the parts would be maintained. This 
equilibrium, then, is disturbed by the action of the 
moon. Indeed we readily see that its action, being 



TIDES. 



235 



oblique to those particles of the sea which are in qua- 
drature with it, and direct to those which are situated 
upon a'right line from the moon to the centre of the 
earth, renders the former heavier, while it lessens the 
weight of the latter. Under the influence of such ac- 
tion, in order that the equilibrium may be restored, it 
is necessary that the waters should become elevated, 
under the moon, in order that the difference in the 
weight may be compensated by the increased height. 
The particles of the sea situated in the corresponding 
point of the opposite hemisphere, being less attracted 
by the moon than the centre of the earth, by reason of 
their greater distance from it, will move less rapidly 
towards that body than will this centre, from which it 
is evident that high water must take place at this point, 
nearly at the same instant as under the moon; and that 
it will be sustained at its elevation by the augmenta- 
tion of the gravity of the columns of particles situated 
in quadrature, and freely communicating with the ele- 
vated mass. 

Let us illustrate this by a diagram. 
Supppose, fig. 39, ABCDEFG 
H the earth, and M the moon. As 
attraction acts in the inverse ra- 
tio of the squares of the distances, 
as already shown, the waters situa- 
ted at Z will be more strongly attrac- 
ted than those at B and at F, where 
the oblique force is resolved into 
two, one acting in the direction of 
the centre of the earth, O, and the 
other in a right line to the centre of 
the moon. The waters at Z, then, 
it is seen, would become elevated. 
Meanwhile the centre of the earth, 
O, being nearer the moon than the waters at N, would 
be more powerfully attracted tkan they, and conse- 



Fig. 39. 






' 




Ib (f 








n 





236 TIDES. 

quently would be drawn more towards that body; or, 
in other words, would be drawn away from the waters 
at Nj which would remain at an elevation; being sus- 
tained, in some degree, too, by the heavier particles at 
B and F, in quadrature; we say heavier, because, 
being obliquely attracted by the moon, as we have 
shown, their gravity is increased. Through the influ- 
ence of these agencies, then, the waters at B and F, 
become depressed towards O, while at Z and N they 
are elevated 

From this it follows that there are formed, upon the 
surface of the ocean, two meniscus shaped masses of 
water, one upon the side next the moon, as shown at 
I Z R, and the other in the opposite hemisphere, at 
L N K, which give to the watery surface of our 
globe the form of an oblong spheriod, of which the 
major axis lies in the right line through the centres of 
the earth and moon. From what has been shown it is 
evident that there would be, at any given point, but 
two tides, or elevations of the water, in each month, 
were it not for the rotary motion of the earth. Let us 
now see what complication this rotary motion of the 
earth adds to the phenomena of the tides. 

By the rotary motion of the earth, upon its axis, the 
most elevated point of the water is removed from di- 
rectly beneath the moon, in the direction of the rota- 
tion; but the water still obeys, for a time, the impulse 
which the attraction of the moon has given it, and in 
consequence continues to rise after it has passed from 
the position directly under the moon, although the im- 
mediate attractive action of that body is then, of neces- 
sity, diminished. The water never attains its greatest 
elevation directly under the moon, nor until after the 
moon has passed the meridian of the place where it 
occurs. In the open seas, where the waters flow with- 
out interruption, the moon is in p at the moment of 
highest water at the p#ints Z and N. It is true, indeed, 



TIDES. 237 

that even after the attraction of the moon has ceased 
its elevating influence, by reason of its having passed 
out of the meridian of the greatest elevation, the as- 
cending movement, which has been communicated to 
the water, still continues, for a time; and this, too, 
even after the moon, from her position, has begun to 
exert the influence of her attraction somewhat against 
such ascension. 

As before stated, when the moon elevates the waters 
at Z and at N they are lowered at B and F, since they 
cannot be elevated at one or more points, without cor- 
responding depressions in others; and reciprocally, 
when they are elevated at B and F, they are depressed 
at Z and N, which points will then be in quadrature. 
But, in consequence of the rotary motion of the earth, 
the moon passes, each day, through the superiour and 
inferiour meridian of each point of its surface; thus 
producing, there, two tides, that is, two elevations and 
two depressions, in about twenty-four hours. 

We have hitherto considered only the action of the 
moon, in producing the phenomena of the tides; but, 
as the sun has also an agency in these, we have next 
to ascertain what this agency is. 

The attractive force exercised by the sun, upon the 
earth, is much superiour to that of the moon; but as the 
distance of the former of these bodies is nearly four 
hundred times greater than that of the latter, the attrac- 
tive power exerted by the one, upon the different parts 
of our planet acts much nearer in parallel lines, and 
consequently approaches much nearer to equality upon 
the whole surface than that of the other. Now, we 
have seen that it is the attraction of the moon, une- 
qually exercised upon different points of the earth's 
surface, which causes the tides; and the action of the 
sun, therefore, which is much more equal upon that 
surface, must consequently be much less efficient in 
producing these results. It has been computed that the 
20* 



*238 



TIDES. 



influence of the sun is about two and one half times 
less than that of the moon. Still it is sufficient to pro- 
duce a flux and reflux of the ocean; so that there are, 
in fact, two tides, one lunar and the other solar; and 
these act with, or antagonistick to, each other, accord- 
ing to the direction of the forces which produce them. 
For instance, when the moon is either full or new, 
that is, when that planet is in the syzygies, A or B, 
Fig. 40. 




fig. 40. the sun, S, and moon are then in the same 
meridian, and as their attractive effects are united in 



TIDES. 



239 



the same line, they snould, as in this position they do, 
produce the greatest possible result — causing the highest 
tides at Z and N, and the greatest depression at H and 
R. On the contrary, when the moon is in quadra- 
ture, the opposite effect is produced. Suppose, fig. 41, 

Fig. 41. 




B N F Z the earth, S the sun, G the moon in one 
part of her orbit, and R the same body in the opposite 
point, or 180° from G. In either of these positions, 
the moon being in quadrature, its agency, and that of 
the sun, in producing tides, are antagonistick to each 
other; the solar influence tending to produce high wa- 
ter at B and at F, while the lunar attraction tends to 
produce that result at Z and N. These conflicting for- 
ces thus partially destroy each other, and the result 
is proportionately diminished. Consequently, as the 
greatest tides known, occur when the " sun, moon and 
earth have the relative positions shown in fig. 40, so 
the least tides are those which occur when these res- 



'240 TIDES. 

pective bodies occupy the relative positions assigned 
them in fig. 41. 

It would seem from this that the instant of high tide 
should be that one when the resultant force of the at- 
tractions of the sun and moon has reached its greatest 
intensity; but we have already seen that this is not so. 
Upon the days of the new moon, when the sun and 
moon exert their combined action in the same right 
line, the instant of the greatest intensity of this action 
is that of their simultaneous passage of the meridian, 
or of mid-day: yet the moment of full tide does not or- 
dinarily arrive until sometime after mid-day. Observa- 
tions and experience have shown that the tide which oc- 
curs upon the day of the new moon is that which was 
produced thirty-six hours previously, by the combined 
action of the sun and moon: and it has been farther 
observed that, at this epoch, full tide always occurs at 
the same hour. From this it has been inferred that 
the interval of time between the instant of the greatest 
attraction of the sun and moon, and that of full tide is 
constantly the same. The second consequence drawn 
from these two facts is, that the action of the sun and 
moon is rendered sensible in ports, and upon coasts, 
by the successive communication of waves and cur- 
rents. 

We have said that, upon the days of new and of full 
moon, the instant when these two bodies exert the 
greatest attractive force is that of the passage of the 
moon through the meridian; and the same is true of 
the first and last quarter. Upon other days, this in- 
stant sometimes precedes and sometimes follows such 
passage; but it never deviates very greatly, because, 
as we have seen, the action of the moon is much 
greater than the sun. These forces, and the retarda- 
tion or the advance of the tide, upon the time of the 
moon's passage through the meridian, vary in propor- 
tion as the sun and moon recede from or approach to 



TIDES. 241 

the earth, as the declinations augment or diminish. The 
flux of the sea is highest, and the reflux lowest at the 
time of the equinoxes, in March and September, be- 
cause at these epochs, all the circumstances tending 
to the production of tides are so combined as to pro- 
duce their greatest possible effect. 

The following are the principal circumstances of the 
phenomena of the tides. The waters of the sea run, 
for about six hours, from south to north, gradually 
swelling or augmenting their height; they then remain 
about one quarter of an hour stationary, and then flow 
from north to south, in the same manner, during the 
next six hours. After a second repose of fifteen mi- 
nutes, they again commence to flow north, when the 
same process is again repeated; and so on, continually. 

The period of the flux and reflux is, in mean time, 
about 12 h 25 m . It is half of the lunar day, which is 
24 h 50 m — the time which elapses between two succes- 
sive returns of the moon to the same point of the me- 
ridian. Thus the sea experiences, then, one flux and 
one reflux, in any given place, as often as the moon 
passes either the superiour or inferiour meridian of 
that place; that is to say, twice in 24 h 50 m . 

These laws of the flux and reflux of the ocean would 
perfectly accord with observed phenomena, if the wa- 
ters of the ocean covered the entire surface of the globe; 
but this is not the case, and therefore it is only in broad 
seas, and far from extensive continents, that the phe- 
nomena are presented such as we have described them; 
and there only because the expanse of waters is suffi- 
ciently extensive for the sun and the moon to exercise 
their attractive powers, in giving motion to the waters, 
without obstruction from contiguous coasts. All these 
phenomena are necessarily modified, in the vicinity of 
continents, by the direction of winds, the situation and 
shape of coasts, inequalities in the depth of water, and 
various other attendant circumstances. 



242 TIDES. 

The tides are very sensibly influential upon the 
courses of large rivers, often reversing their currents, 
for great distances; and in some cases the waters of 
rivers rise, regularly, with every tide, at the distance 
of two hundred leagues from their mouths; being dri- 
ven back by the tidal wave of the ocean. 

Lakes, or inland seas, are not subject to tides, being 
too small, in extent of surface, for the action of the 
moon to become sensible, during the short time which 
that planet employs in passing over them. 

The want of tides in the Mediterranean and Baltick 
seas is accounted for by the fact that the straits by 
which these two great lakes communicate with the 
ocean are so narrow that they cannot receive, through 
them, in so short a period, a sufficient quantity of wa- 
ter to sensibly elevate their surfaces. 

In the West India Islands the tides are compara- 
tively trifling; rarely rising more than from twelve to 
fifteen inches. This anomaly at first appears the more 
remarkable, as the regions in question are near the 
equator, and must, therefore, be subject to a very pow- 
erful attraction. But we readily perceive that the wa- 
ters should not be much elevated, in the vicinity of 
these islands, the moment we recollect that, the earth 
turning from west to east, the flux, progressing in a 
contrary direction, arrives, like an immense wave, 
and breaks against the coast of America, which arrests 
it there, and prevents its passing, with the moon, to 
the Pacifick Ocean. The trade winds, also, which 
blow constantly from east to west, oppose the reflux, 
returning from the west. 

The same two causes, combined, produce a very 
remarkable effect in the Gulf of Mexico. The winds 
and tides continually press the waters into this vast 
indentation of the coast, until they accumulate there, 
above the general level, and, by their incessant action, 
maintain these waters at this increased elevation. Thus 



LATITUDE AND LONGITUDE. 243 

suspended, and unable, by their weight, to overcome 
the obstacles to their return, these waters flow round 
the west coast of the island of Cuba, and thence north- 
ward, towards the coast of America, forming that re- 
markable current known as the Gulf Stream. 

The particles of the atmosphere being endowed, in 
a still greater degree than those of water, with mobi- 
lity, and at the same time air being lighter than water, 
it would seem that this gas should obey the combined 
action of the sun and moon, and thus experience aerial 
tides. Yet it has been supposed, in spite of these 
facts, that proof is furnished that there are no such 
tides, because the barometer gives us no indication of 
their existence, by noting the elevations and depres- 
sions of the atmosphere. Yet it is easy to see that the 
barometer could not give us the least indication of such 
tides, but must remain insensible to them; since the 
columns of air, however different their height, would 
remain of the same weight, because the direct effect of 
a tide is, as w 7 e have shown, to maintain the equili- 
brium by compensating, in height, for the diminution 
of gravity. 



CHAPTER FOURTEENTH. 

DETERMINATION OF LATITUDE AND LONGITUDE. 

To determine the position of a given point, upon 
any surface, whatever, it is necessary to know the dis- 
tance of such point from two fixed lines, upon the same 
surface. These two lines may be variously disposed; 
but their situation, upon the surface in question, should 
be invariably fixed. Yet, by reason of the facilities 
which it gives both to construction and to computation, 
such lines are usually disposed so as to form right an- 



244 



LATITUDE AND LONGITUDE. 



gles with each other, instead of any other angle. It 
follows, therefore, that the process which we employ- 
to determine the position of different points of the sur- 
face of the earth is absolutely the same as that of 
which we make use to determine the positions of the 
stars. It is enough if we know the parallel upon which 
the point in question is situated, and the part of that 
parallel which it occupies; for, to know these, is to 
know the latitude and longitude of the place. 

And first of latitude. This may be determined by 
simply taking the height of the pole above the horizon; 
for it is always equal to this height. This has already 
been shown, in chapter third, pages 73 and 74; but 
we will repeat the diagram, and, summarily, its de- 
scription, here. Suppose the point C, fig. 42, is remo- 
Fig. 42. 




ved 30°, for example, towards the north pole; then, 
its zenith will be C F; the great circle H O R will be 
its horizon; the place of the equator, E O Z, will be 
elongated 30° from the zenith, F, and consequently 
60° degrees from the horizon. The pole, P, will be 
elevated 30°, measured by the angle H C P. 



LATITUDE AND LONGITUDE. 245 

This, then, gives us our latitude: but as there is, in 
the southern hemisphere, a circle which presents the 
same circumstances, it becomes necessary always to 
designate whether a given latitude is north or south. 
Of course there are as many of these circles, or paral- 
lels of latitude, on each side of the equator, as there 
are points from the equator to the pole. 

The determination of longitude is attended with 
much more difficulty. To ascertain this, we measure, 
in degrees of the equator, the distance which separates 
the meridian of the place to be determined, from some 
other known meridian. Now this distance may al- 
ways be determined, with certainty, provided we know 
the time at the point where the observation is made, 
and also the time at the known meridian. The reason 
for this we will explain. As each point of the earth's 
surface, by reason of the rotary motion of our globe, 
upon its axis, describes the circumference of a circle, 
or 360° in 24 h , then it, of course, describes 15° in l h , 
because 15 is the 24 th part of 360. Then it follows 
that, if the two places in question are separated from 
each other by 15° of longitude, the most western one 
will not have the sun in its meridian, which is the mo- 
ment of noon, until one hour after the other. The 
more eastern of the two, then, will have 12 o'clock, at 
noon, at the same instant when it is 11 o'clock in the 
morning at the other place, 15° west of the first. If 
the distance which separates the two places be 30°, 
then the difference, for the reasons given, will be two 
hours, and so on; and for odd degrees, and parts of 
degrees, in the same proportion — each degree being 
equivalent to four minutes of time. If the difference 
of time, then, can be known, at the different points, 
nothing is easier than to know the difference of longi- 
tude : and, reciprocally, if the longitude be known, the 
time is disclosed by that. 

21 



246 LATITUDE AND LONGITUDE. 

All the difficulty, then, in the case, arises in ascer- 
taining the difference of time. For determining this, 
a great variety' of methods are employed A- 
quite impossible for us to make all these known, here, 
we shall only speak of a few of them. 

The exact instants of eclipses of the moon, of the 
sun, the occultations of stars, by the moon, the eclipses 
of the satellites of Jupiter, <kc. occurring under the 
known and given meridian, are, to aid the purposes of 
navigation, ore. carefully calculated and published, 
several years in advance. Now, suppose a navigator, 
or traveller upon land, situated at any distance, what- 
ever, to the east, or the west of this given meridian, 
observes one of these eclipses or occultations, and 
recurring to his tables of calculations, he there finds 
the time, at the given meridian, and the difference of 
time from that where he is, of course gives him 
his longitude. He has only to re-member that if it is 
later at the place of his observation than at the given 
meridian, then his longitude is west; and if earlier, 
then he is east of the given meridian, or in east lon- 
gitude. Whenever the sky is unclouded, we are al- 
ways enabled to recur to this species of observations, 
the phenomena, of that kind, being much more nu- 
merous than the days of the year: nor is there any 
necessity. for this purpose, of very powerful instru- 
ments; and, indeed, such cannot be employed, at sea, 
on account of the rolling of the ship. 

Marine time pieces, called, also, chronometers, are 
of great assistance in determining longitude. These, 
from their peculiar construction, and excellant me- 
chanical execution, preserve their uniform rate of 
motion, with great exactness, in despite of the varia- 
tions of temperature, and the inevitable jolts and 
shakings of long voyages. These, on departing upon 
a voyage, are carefully set to the exact time of that 



LATITUDE AXD LONGITUDE. 247 

meridian to which the navigator intends to refer his 
longitude. By this method he has always at command 
the means of knowing the difference of time between 
where he is, and at the assumed meridian; for, while 
observation gives him that where he is, his chronome- 
ter points him to the other; and a comparison of these 
gives him his longitude. 

This last method of resolving the important prob- 
lem in question is so simple, and so easy of accom- 
plishment, that it would be useless to resort to any 
other if we were able to depend, with perfect confi- 
dence, at all times, upon the exactness of chronome- 
ters. But this, unfortunately, cannot always be done. 
Still, the progress of modern ingenuity has carried 
the construction of these time pieces to a degree of 
perfection which, at one time, no person even dared 
to hope would ever be attained. 

Some just idea of this perfection may be formed 
from the following extract: "If permitted, the author 
of this work will share with the reader the pleasure 
and surprise which he experienced, after a long voyage 
from South America to Asia. The chronometer in, 
his pocket, and those belonging to the ship announced, 
one morning, that a certain tongue of land, indicated 
upon the chart, should then be situated fifty miles to 
the east of the ship. Judge, then, of the excellence 
of these instruments from the fact that, one hour after, 
the morning fog having disappeared, the man at the 
mast head gave the joyful cry of land! land! directly 
ahead! thus confirming the prediction of the chro- 
nometers, within about one mile, after a voyage of 
such enormous length! It is at such a moment, and 
under such circumstances, that we are most deeply 
impressed with profound admiration of human genius. 
When we compare the dangers of ancient naviga- 
tion with the safe and confident progress of ships, at 



248 LATITUDE AND LONGITUDE. 

the present day, it is impossible to deny the immense 
advantages of modern discoveries and ingenuity. If 
the running of these little instruments had been 
altered, in the least, during this period of several 
months, their prediction would have been more inju- 
rious than useful; but, by night as by day, in tempest 
as in calm, in heat and in cold, the beats of these 
time pieces succeeded each other, with undisturbed uni- 
formity, keeping, if I may so speak, an exact account 
of the motions both of the heavens and the earth; and, 
in the midst of the waves of the ocean, which retain 
no trace, they always marked the exact situation of the 
ship whose safety had been confided to them, as well 
as the distance she had run, and the space she had 
still to overcome, for the completion of her voyage." 

We have said that these marine time pieces arc 
to be carefully set to the exact time of that meridian to 
which the navigator intends to refer his longitude; and 
that this is to be done . at the commencement of his 
voyage. Now, it will readily occur to the mind that 
most voyages may commence from ports that are not, 
themselves, upon this meridian. Such, in fact, is the 
'case: but it will be recollected that if the longitude of 
any place be known, in degrees, minutes and seconds, 
it is easy, by converting these into time, to know the 
hour, minute and second of time upon the first meri- 
dian. Then, from whatever port a vessel may com- 
mence her voyage, the longitude of that port being 
previously known — and it is usually recorded in nau- 
tical tables — the navigator readily finds the time on 
his first meridian, by the method we have indicated; 
and to this time, and not to that where he is, he sets 
his chronometer. The same means are resorted to, so 
far as practicable, when, from any cause, the time 
piece shall have become deranged or stopped, upon 
a voyage: so that there is no necessity of actually 



LATITUDE AND LONGITUDE. '249 

going to the first meridian, as might be supposed, to 
obtain the true time of that meridian. 

The meridian to which each astronomer refers his 
observations is entirely arbitrary, and diners, with dif- 
ferent people. Some of the chief nations assume, as 
this known meridian, that which passes through their 
respective capitals: thus French books, maps, charts, 
&c. have their longitude reckoned from the Royal 
Observatory, at Paris: the English, in like manner, 
from Greenwich Observatory, near London; while 
Ame: we have no observatory, in the United 

States/ are obliged to assume the building known as 
the Capitol, at Washington. 

There are evils attending this multitude of meridians, 
or starting points, which the celebrated Laplace pro- 
posed to remedy, by an agreement, among ail nations, 
->ume some prominent object, in nature, as the 
Peak of Teneriffe, (one of the Canary Islands) as a 
common meridian: but unfortunately the suggestion 
has not been adopted. Indeed Ptolemy, a very an- 
cient astronomer, did assume the most western of the 
Canary Islands, as his first meridian, this being, in 
his day, the western limit of known countries: and 
for a long time his example was followed: but the prac- 
tice has fallen rapidly into disuse, since the discovery 
of America, and is now entirely abandoned. 

have spoken of eclipses, occultations, 6zc, cal- 
culated in advance of the time of their occurrence. 
These are arranged in tables, with a multitude of 
other no less important calculations, and published in 
a species of almanack, some three years before their 
date. This is done in order that every navigator, and 
scientifick man, may always be in possession of 
. when the day of their use arrives, let him be in 
what part of the world he may. 

One of these works was commenced at Paris, by 
Picard, in the Year 1679, and has been issued, regu- 
21* 



250 LATITUDE AND LONGITUDE. 

larly, every year since that time. After this had been 
published eighty-eight years, namely, in 1767, Maske- 
lyne commenced a similar work, at London, which is 
still regularly published, every year. There is an- 
other of these works, of great merit, and accuracy, 
published at Berlin. Navigators and others, from the 
different nations of the continent of Europe, make 
use of either the Paris or Berlin tables, mostly the 
former; while the English, of course, employ those 
of London. These last, being in the language of our 
own country, are generally, though not wholly, used 
by navigators of the United States. 

There are numerous other astronomical tables, such 
as tables of each different planet, &c. which it has 
cost immense labour to calculate, and which have been 
mostly produced by the French astronomers. These 
being indispensable, in practice, have been generally 
copied into the books of the English and other nations. 

We have seen, in chapter 9th, page 169, that de- 
grees of latitude, which are measured from the equa- 
tor towards either pole, are not of equal length; but 
that, by reason of the flattening of the earth, at the 
poles, degrees of latitude increase in length, as we 
approach either pole. But, with degrees of longitude 
the reverse of this is true. While latitude is reckoned 
upon lines from the equator to the poles, longitude is 
counted, as already shown, upon lines at right angles 
to these, called parallels of latitude. It is evident, 
from this, that degrees of longitude must vary greatly 
in. their length. The longest is at the equator, being 
sixty geographical miles; and from this they decrease 
in length, as the latitude increases, but not in the same 
proportion. That the length of these degrees may 
readily be known, the following table is given, show- 
ing the length, in geographical miles and one hun- 
dredth parts of a mile, of every degree of latitude, 
from the equator to the poles. 



LATITUDE AXD LONGITUDE. 



251 



TABLE SHOWING THE NUMBER OF MILES AND HUN- 
DREDTHS OF A MILE, IN A DEGREE OF LONGITUDE, 
IN ANY GIVEN DEGREE OF LATITUDE. 



Degree. 


Miles. Dec. 








Miles. Dec. 


1. 


59 .99 


31. 


51 .43 


61. 


20 .09 


O 


59.96 


32. 


50 .88 


62. 


28 .17 


3. 


59 .92 


33. 


50 .32 


63. 


27 .24 


4. 


59 .85 


34. 


49 .74 


64. 


26 .30 


5. 


59 .77 


35. 


49 .15 


6b. 


25 .33 


6. 


59 .67 


36. 


48 .54 


66. 


24 .41 


7. 


59 .56 


37. 


47 .92 


67. 


23 .44 


8. 


59 .42 


38. 


47 .28 


68. 


22 .48 


9. 


59 .26 


39. 


46 .63 


69. 


21 .50 


10. 


59 .09 


40. 


45 .97 


70. 


20 .52 


11. 


58 .89 


41. 


45 .28 


71. 


19 .53 


12. 


58 .69 


42. 


44 .59 


72. 


18 .54 


13. 


58 .46 


43. 


43 .88 


73. 


17 .54 


14. 


58 .22 


44. 


43 .16 


74. 


16 .53 


15. 


57 .95 


45. 


42 .43 


75. 


15 .52 


16. 


57 .67 


46. 


41 .68 


76. 


14 .51 


17. 


57 .33 


47. 


40 .92 


77. 


13 .50 


18. 


57 .06 


48. 


40 .15 


78. 


12 .48 


19. 


56 .73 


49. 


39 .36 


79. 


11 .45 


20. 


56 .38 


50. 


33 .57 


80. 


10 .42 


21. 


56 .02 


51. 


37 .76 


81. 


09 .33 


22. 


55 .63 


52. 


36 .94 


82. 


08 .35 


23. 


55 .23 


53. 


36 .11 


83. 


07 .32 


24. 


54.81 


54. 


35 .27 


84, 


06 .28 


25. 


54.38 


55. 


34.41 


85. 


05 .24 


26. 


53 .93 


56. 


33 .55 


86. 


04 .20 


27. 


53 .46 


57. 


32 .63 


87. 


03 .15 


28. 


52 .96 


58. 


31 .79 


88. 


02 .10 


29. 


52 .47 


59. 


30 .90 


89. 


01 .05 


30. 


51 .96 - 


60. 


30 .00 i 


90. 


00.00 



252 OF THE ATMOSPHERE, ETC. 

CHAPTER FIFTEENTH. 

OF THE ATMOSPHERE, IX ITS ASTRONOMICAL RELA- 
TIONS. 

The atmosphere is that gaseous envelope which, 
upon every side, surrounds our globe. Before inves- 
tigating the influence which this exercises over the 
observations of astronomical phenomena, let us spend 
a few minutes in an examination of some of its own 
properties. 

And, first, what is the height of the atmosphere? 
This question has been resolved by the aid of one of 
the most precious instruments pertaining to physicks, 
namely, the harometer, an instrument for measuring 
the weight of the atmosphere. We know that, in car- 
rying the barometer to positions having different de- 
grees of elevation, it should indicate distinct differen- 
ces in the weight of the column of air above it, at 
these different elevations; and a simple question of 
proportion would be sufficient to disclose to us the ab- 
solute height of the atmosphere, if it had, throughout 
all its mass, the same density. But it being extremely 
compressible, the inferiour layers, next the earth, 
which have to support the weight of all the superiour 
layers, are necessarily most compressed, so that the 
density of an atmospherick column constantly dimi- 
nishes, from the surface of the earth, upward, to the 
very surface or limit of the atmosphere itself. 

Calculations have shown that, supposing the tem- 
perature of the air the same, throughout, the height 
of the mercurial column, in the barometer, diminishes 
in arithmetical progression, when the elevations above 
the level of the sea increase in geometrical progression. 
But it is necessary, in this experiment, to have strict 
regard, both to the temperature, and the hygroscopick 
state of the atmosphere. In this way measures have 



OP THE ATMOSPHERE, ETC. 253 

been obtained by which it is supposed that the height 
of the atmosphere is between 30 and 40 miles: its 
volume one 29th that of the earth, and its weight only 
forty-three one-thousandths that of our globe. 

But what is there beyond the atmosphere? Is there 
some fluid existing there, or are the planetary spaces 
an absolute void? We know not, in truth, how this 
question, in this particular form, could so long have 
occupied the attention of the learned; for really there 
is nothing of it. How can the celestial spaces be an 
absolute void, when they are filled with light? Now, 
whatever opinion we adopt upon the nature of this 
agent, either that it is a real emanation of the sub- 
stance of luminous bodies, or a fluid put in motion by 
these, it is very evident that, upon either hypothesis, 
the absolute void can have no existence. Void of all 
resisting agents the regions in question may very well 
be, and probably are.* 

But it is particularly in relation to the action which 
the atmosphere exercises upon the luminous rays 
which pass through it, that this substance demands 
our attention, here. 

We have seen, at the commencement of this work, 
the modifications which light experiences in passing 
out of one medium into another: how it is refracted, 
and how its rays are decomposed. It is to this pro- 
perty of light that we owe the variegated and party- 
coloured appearance of the clouds which adorn the 
horizon at the rising and setting of the sun. If is to 
this that we are indebted for our preservation, each 
day, from instantaneous transition from darkness to 
light, and from light to darkness, through the inter- 
vention of morning and evening twilight. These phe- 
nomena vary with the diversity of seasons and of 
place. It has been calculated that, through the effect 



* See appendix. 



254 OP THE ATMOSPHERE, ETC. 

of atmospherick refraction, daylight does not entirely 
cease, to us, until the sun is 18° below the horizon. 

One of the effects of atmospherick refraction is to 
cause the apparent positions of the stars to vary. In 
fact, the different layers of the atmosphere, augment- 
ing in density in proportion as they approach the sur- 
face of the earth, may be considered, in relation to 
each other, as different mediums. The luminous rays 
which traverse these deviate more and more, from a 
right line, in passing from one to the other; and as 
the density augments insensibly, this deviation, in- 
stead of breaking the rays into a series of right lines, 
causes them to follow a curved line, the concavity of 
which is turned towards the surface of the earth. It is 
now easy to conceive how the effect of this refraction 
is to show us objects above their real position: because 
we always view objects in prolongation of the direc- 
tion by which the ray of light penetrates the eye, in- 
stead of following that ray, in its curvature, through 
the atmosphere. In this way, refraction augments 
the apparent heights of the heavenly bodies, when 
viewed in any direction, except directly in the zenith. 
Here refraction is nothing; and it augments with 
every increase of distance from this point to the hori- 
zon. 

It is proper to explain, here, a phenomenon which 
the moon presents, at the horizon, and which is some- 
times designated the horizontal moon. This planet 
there assumes an elliptical form, and appears much 
greater, and less brilliant, than when it is in the 
meridian. 

And at first, that we may commence with the cir- 
cumstances most easily explained, it is evident that if 
the light of the moon is less vivid at the horizon than 
in the meridian it is because the luminous rays which 
it sends to us have to traverse an atmospherick layer 



OP THE ATMOSPHERE, ETC. 



255 



of much greater thickness and density in the former 
than in the latter position. 

To illustrate this, and also the principle of refrac- 
tion just laid down, suppose, fig. 43, C E D the sur- 
Fig. 43. 




face of the earth; ABC D the earth's atmosphere; 
H the moon in the horizon, I the same planet at a 
greater elevation, and K still the same planet, in the 
meridian of an observer placed upon the earth, at E. 
Here it is manifest that the rays from the moon, at H, 
pass through the greatest distance of atmosphere, and 
those from K through the least. The refraction, or 



250 OP THE ATMOSPHERE, ETC. 

bending of the rays from H is seen to be greatest, 
beginning as they enter the atmosphere, at f g, while 
at K they are not turned from right lines. At H, too, 
the concave side of the curve followed by the rays is 
seen to be turned, as before stated, towards the sur- 
face of the earth, by which means the spectator, at E, 
woxild see the moon in the direction f g, while it was 
really at H. From this diagram it is manifest that the 
rays from the moon at H must be more feeble and 
discoloured than from the same body at a greater ele- 
vation, particularly when we recollect that these rays 
pass a long distance near the surface of the earth, and 
consequently must be obstructed by much vapour. 

As to the apparent dimensions of the disk of the 
moon, it is a phenomenon which has greatly perplexed 
philosophers. The moon is more distant from us, at 
the horizon than in the zenith, by half the diameter of 
the earth — a difference, it is true, so small that it can- 
not produce, upon the apparent dimensions of this 
planet, any sensible effect; and most certainly not the 
effect to enlarge it. What, then, can produce this ap- 
pearance? Gassendi supposed, as the moon is less 
brilliant at the horizon than in the meridian, that we 
enlarge the pupil of the eye more, while looking at it 
in the former position, and that this would cause it to 
appear larger. But, in order that this conclusion may 
be admitted, it is necessary that the enlargement of 
the pupil of the eye shall augment the size of the 
image painted upon the retina: which is contrary to. 
all the principles of opticks, and is contradicted by 
the most exact and careful experiments. Other phi- 
losophers have thought, and perhaps with more rea- 
son, that the moon appears larger at the horizon than 
in the meridian, because we suppose it there at a 
greater distance from us. Indeed they assume that 
two considerations enter into the act of vision, namely, 
the angle under which we view the object, and the 



OP THE ATMOSPHERE, ETC. 257 

distance at which we suppose that object to be situated. 
This judgement or opinion, which we form uncon- 
sciously, directly influences the impression produced 
by the image; and so true is this result, that we readily 
appreciate the stature of two men, for instance, who 
are at very unequal distances from us, and who are 
therefore seen under very different angles. Another 
experiment is no less striking. If we place an object 
upon a horizontal plane, and the eye in the prolonga- 
tion of that plane, and thus obtain such a view of the 
object as to see two images of it (which is easily done, 
by pressing the under eyelid and the eye gently up- 
ward, with the finger) the two images will be of dif- 
ferent magnitudes, the nearest being the smallest; and 
this will continue to decrease, as it is made to approach 
the eye. 

Now, continue the partisans of this explanation, the 
moon, at the horizon, appears to us to occupy the in- 
feriour part of the immense celestial dome, or vault, 
and its distance seems to us greater, there, than when 
at the summit of this dome, that is to say, in the ze- 
nith. Again, in the horizon, the moon's apparent 
distance is still farther increased by those comparisons 
which intermediate objects furnish. In this way the 
errour of judgement in regard to distance and its in- 
fluence modifies the impression produced by the image 
itself, and causes us to see the moon larger than its 
size and distance will warrant. 

Such is the prevailing explanation of this phenome- 
non, at the present day. But, without contesting the 
principles upon which it rests, we think, even if the 
assigned cause concurs to produce the phenomena of 
the horizontal moon, that still this is not the sole cause, 
but that there is another, of which the action and the 
effects are even more evident; and this is refraction. 
The luminous rays, emanating from the extreme bor- 
ders of the moon's disk, through the agency of refrac- 
22 



258 THE HARVEST AND 

tion, arrive at the eye under. a more open angle, thus 
causing an apparent enlargement of the moon itself. 

In regard to the figure which the horizontal moon 
assumes, this, also, is the effect of refraction. The 
moon, we have said, in this case assumes an elliptical 
form. This, by the laws of refraction, should be so; 
for the rays emanating from the horizontal diameter, 
penetrating into the atmosphere under the same angle, 
are equally refracted; while with those which emanate 
from the vertical diameter, this is not so; for those 
which come from the upper extremity, entering into 
the atmosphere in a more oblique direction than those 
from the inferiour border of the disk, are differently 
refracted: and the effect of this inequality of refrac- 
tion should be to change the apparent figure of the 



While treating of the moon we will say a few words 
upon two other phenomena which this planet presents. 
Twice in each year this planet, when at her full, rises 
almost at the same time for the period of nearly one 
week: and it is known at the first of these periods as 
the Harvest moon, and at the other as the Hunter's 
moon. 

The moon, as we have seen, moves in its orbit from 
west to east. When the earth, then, by the effect of 
its daily revolution upon its axis, returns from one 
meridian to the same again, the moon, which has in 
that time passed over a little more than one thirtieth 
part of her orbit, and in the same direction, is then 
found to be twelve degrees and some minutes in ad- 
vance of its position the previous day. 

The plane of the equator being perpendicular to the 
axis of rotation of the earth, it is evident that all the 
parts of the equinoctial circle make equal angles with 



THE HUNTER S MOON'S. 



259 



the horizon, both east and west; and that, in equal 
times, there are always equal portions of this rising 
and setting, at all times of day, and at every season 
of the year. If, then, the moon moved always in the 
equinoctial plane, and if it outran the sun 12° 11' 
each day, the distance it moves in its orbit, then this 
planet would rise and set each day just fifty minutes 
later than on the preceding day. 

But its orbit is considerably inclined to the plane of 
the equator, and is situated much nearer in the plane 
of the ecliptick: so near, indeed, that, for the present 
illustration, we may consider them the same. Now, 
the different parts of this plane, which is oblique to 
the axis of the earth, make different angles with the 
horizon, both east and west. The parts which rise 
with the least angles are those which set with the 
greatest, and reciprocally. When this angle is least, 
then, in equal times, of course, there rises above the 
horizon a greater portion of the ecliptick than when 
the angle is greatest. To illustrate this, figs. 44 and 

Fig. 44. Fig. 45. 




45, suppose L the latitude of London, A B the horizon 
of the place, F P the axis of the world, E e the equi- 
noctial, and K k the ecliptick. In consequence of 
the oblique position of the sphere, in the latitude of 
London, that is, in consequence of its high latitude, 



260 THE HARVEST AND 

the ecliptick has a high elevation above the horizon, 
and makes, in fig. 44, the angle A V K, of about 
sixty-two and a half degrees, when the sign Cancer, 
S> is in the meridian, and that of the Balance, =2=, is 
just rising in the east. But when the other part of 
the ecliptick is above the horizon, that is to say, when 
the sign Capricornus, Y3, is in the meridian, and Aries, 
<P, is rising in the east, the ecliptick makes but the 
very small angle shown at k V A, fig. 45, of about 
15°, being 47 1 ° less than the former. 

Thus, supposing the celestial sphere to turn, as it 
seems, upon the axis F P, a greater part of the eclip- 
tick will rise above the horizon, in a given time, when 
the sphere has the position shown in fig. 45, than when 
it has that shown in fig. 44. 

In north latitudes it is when Arias, <p, is rising and 
the Balance, =£=, is setting, that the ecliptick makes 
the smallest angle with the eastern horizon; and it 
makes the greatest, on the contrary, when the Balance, 
=£*, is rising, and Aries, SP> is setting. From the 
rising of Aries, °p, to that of the Balance, =^, a period 
of twelve sidereal hours, the angle augments; and it 
decreases during the succeeding half of the sidereal 
revolution, or from the setting of °p, to that of =2=. 
Thus, then, in and near <p> the ecliptick rises more 
rapidly above the horizon, while at =£= its elevation is 
less rapid. 

But in the parallel of London the ecliptick rises as 
much towards Pisces, X, and Aries, SP? in two hours, 
as the orbit of the moon in six days; while it is in 
these signs its risings are retarded only two hours in 
six days, that is to say, twenty minutes per day, mean 
time; but the moon, fourteen days after, enters into 
the Virgin, fl£, and the Balance, =2=, which are oppo- 
site to X and SP; and while it is in these signs its 
risings, from day to day, are retarded about one hour 
and fifteen minutes. Through the succeeding signs 



the hunter's moons. 261 

Taurus, tf, Gemini, n» Cancer, <5, Leo, £i, Virgo, 
nd Libra, =2=, the angle formed by the ecliptick, 
with the horizon, augments when they are rising and 
diminishes when they are setting. Thus the risings 
of the moon are more and more retarded, while that 
planet is in these signs, and its settings follow the con- 
trary progression: while the differences of rising grow 
less and less, through the other six signs, namely, 
Scorpio, 111, Sagittarius, /, Capricornus, Y3i Aqua- 
rius, £?, Pisces, X, and Aries, C P- 

But the moon makes the tour of the ecliptick in 
27 d 8 h , and takes 29 h d to return to the same point; so 
that it is, during each lunation, in Pisces, >t, and 
Aries, T> at least once, and sometimes twice. 

Each new moon occurs in 1 the same sign, and each 
full moon in the sign opposite, because, in their inter- 
val the moon runs exactly once over the ecliptick. 
Now, as the full moon rises at the same time that the 
sun sets, for the reasons that, 1st., when one point of 
the ecliptick sets the opposite point rises, and 2d., 
the moon is full only when in the point of the eclip- 
tick opposite to the sun, therefore the moon always 
rises near the setting of the sun, under the parallel of 
London, during the week that it is full. But while it 
elongates itself, in reference to the ecliptick, from con- 
junction or opposition, the sun passes to the following 
sign, in 27 J days. The moon, during the same time, 
surpasses its revolution, and advances more than the 
sun, during the interval of more than two days, be- 
yond this period, before it can enter into opposition 
or conjunction again. We see, then, that there can- 
not be, in any one point of the ecliptick, whatever, 
more than one conjunction or one opposition. This 
motion may be compared to that of the hands of a 
clock, which are never move than once in 12 hours, 
either in conjunction or opposition in the same part of 
the dial over which they both move. 
22* 



262 THE HARVEST AND 

Now, as the moon is not full, as we have seen, ex- 
cept when it is in opposition with the sun, and as this 
last is not in the signs Virgo, HJ?., and Libra, £=, ex- 
cept in autumn, the moon cannot be full, in the oppo- 
site signs, namely, Pisces, >£, and Aries, SP? except in 
these two months. There cannot be, then, in any year, 
more than two full moons which rise, during the period 
of a week, almost at the same time that the sun sets. 

Whenever the moon is in Pisces, X, and Aries, T> 
it rises, indeed, almost at the same hour, but this phe- 
nomenon passes without notice, if it is not full there. 
Thus, in winter, these signs rise about mid-day, and 
the moon being then in quadrature, is not noticed. In 
the spring the sun and moon are in these signs, and 
in conjunction, so that the rising of the moon is unob- 
served. Finally, in summer the rising of the moon, 
in quadrature, takes place at midnight — a period at 
which astronomers, alone, would be likely to observe 
it. It is, then, only in autumn, that the full moon, as 
we have said, rises in these signs, at the setting of the 
sun; and it is this fact which has attracted so much 
attention to the phenomenon. 

These phenomena are equally regular, in their occur- 
rence, upon one side of the equator as upon the other; 
but in south latitudes the seasons are the opposite of 
those at the north. Thus the full moons of spring, 
upon one side of the equator, are precisely in the 
signs of the full moons of autumn upon the other side. 

Reciprocally, in spring the full moons present, at 
their setting, the same phenomena which the full 
moons of autumn exhibit at their rising. 

We have supposed, thus far, for the purpose of 
greater simplicity, that the plane of the lunar orbit 
coincides with that of the ecliptick; but we know that 
these two planes make, with each other, an angle of 
from 5° to 5° 18', cutting each other in the line of the 
nodes. Now the moon passes these at least twice, and 



the hunter's moons. 2(53 

often three times in the interval of two changes. 
Indeed, as the moon gains almost a sign in the inter- 
val between two successive changes, if it pass through 
one node at the epoch of the change, or near it, it 
will return to this again, after having passed the other, 
before the succeeding change. Now, while north of 
the ecliptick, this planet rises earlier and sets later 
than if moving in the plane of the ecliptick; and 
while south of that plane, the result is the contrary of 
this. But the retrograde movement of the nodes 
causes this difference to vary. For, when the ascend- 
ing node is in Aries, % the southern half of the 
moon's orbit makes, with the horizon, an angle of 
5h° less than the ecliptick makes with this plane, 
when Aries, <p, rises, in northern latitudes: this is 
why in Pisces, }£, and Aries, c p, the moon rises with 
less variation of time than it would if it moved in the 
plane of the ecliptick. But the descending node, in 
its turn, arrives at Aries, T? after nine years and 114 
days, when the angle which the orbit of the moon 
makes with the horizon is greater by 151 J°, from 
whence it follows that the moon has longer periods of 
time, between its risings, in Pisces, X, and Aries, °p, 
than if it moved in the plane of the ecliptick. Thus 
it is seen that the phenomena of the Harvest, and the 
Hunter's moon are not, in all years, equally remark- 
able; they pass from their maximum to their mini- 
mum in the period of nine years and a half. 

The full moon of winter is also elevated above the 
ecliptick, as the sun is in summer; and it should, 
therefore, remain longer above the horizon; and, recip- 
rocally, in summer its stay is contracted, as the sun's 
is in winter. It follows from this, that the polar cir- 
cles, which have the sun 24 hours above the horizon, 
and 24 hours below this circle, should also have one 
full moon which should remain 24 hours above, and 
another 24 hours below the horizon. These two full 



264 OP DAYS AND SEASONS. 

moons are the only ones which present these pheno- 
mena; all the others both rise and set. 

The poles have, as we shall see, in the ensuing 
chapter, a day of six months, and a night of the same 
duration, (without regard to the modifications produced 
by refraction,) upon this distribution of light and dark- 
ness. Now, as the full moon, as before stated, is 
always in opposition to the sun, therefore, when the 
sun is above the horizon, the moon, at the time of its 
opposition, or full, is below this plane: it is, then, in- 
visible, half the year. But when the sun has de- 
scended below the horizon, the full moons are visible 
in such places as the sun's rays do not enlighten. 
Thus the poles, which are deprived of the moon, in 
summer, that is to say, when they have the sun, have 
it again enlightening them, in winter, when the sun 
has sunk below their horizon. They are therefore 
rarely in great obscurity; since they enjoy so much of 
the light of the moon, as compensation, in part, for 
the long absence of the sun. 



CHAPTER SIXTEENTH. 

OF DAYS AND SEASONS. 

We have already seen that if the axis of rotation 
of the earth was perpendicular to the plane of the 
ecliptick, the days and nights would have the same 
duration, in all parts of the earth; but the inclination 
of these two planes is 23° 28': and it is this inclination 
which produces the diversity of seasons, and of days. 

It is easy to comprehend the nature and variety 
which the phenomena of day and night present, upon 
different points of the earth's surface. 

At Paris, for example, the latitude is about 48°. 
We should have, then, fig. 46, for the zenith, Z 0: 



OF DAYS AND SEASONS. 



. 



H h would be the horizon, P p the line of the poles, 



Fig. 46. 




and E e the equator. When 
the sun, S, is in the plane of 
the equator, it will describe 
the circle E e, which the ho- 
rizon, H A, divides into two 
equal parts, thus giving the 
sun an equal time above and 
below the horizon, and there- 
fore causing the days and 
nights to be of equal length. 
But when the sun shall have 
declined towards the south pole 23° 28 , bringing it to 
the tropick of Capricorn, "£?, it will describe the circle 
:cle is divided by the horizon, H A, into 
two unequal parts, of which the greater is below this 
plane; and the nights will consequently be longer than 
the days. Finally, six months from this time, when 
the sun shall have attained a northern declination of 
•23 : 28 . which it does on arriving at the tropick of 
Cane ill describe the circle S ' «, which is 

also divided unequally by the horizon, making the 
days longer than the nights, at this season. 






Let us now observe the na- 
ture of these phenomena in 
the equatorial regions, repre- 
sented by fig. 47. To an 
inhabitant here, the zenith, 
O Z, coincides with the equa- 
torial plane, E e, and the ho- 
rizor. :h the axis of 

the poles, P p. Now the sun, 
whether it be at S, S', or S", 
that is to say, whether it be 
at the equator or at either 
tropick, always describes circles which are divided 
into two equal parts, by the horizon; and consequently 



266 



OP DAYS AND SEASONS. 




the equatorial regions have always days and nights of 
equal duration. 

The polar regions, on the 
contrary, fig. 48, have the 
line of the zenith, O Z, co- 
inciding with that of the 
poles, P p, and their hori- 
zon, H h, is conlounded 
with the equator, E e. When 
the sun, S, is in the plane of 
the equator, it describes the 
circle S H, which is that of 
the horizon, and one half of 
the sun's disk, if there were 
no refraction, would be above, and the other half 
below this plane. But when the sun, S", has attained 
the tropick of Cancer, 55? it will describe the circle 
S" N, which is entirely above the horizon, while at 
the tropick of Capricorn, K3> it describes the circle 
S' M, which is wholly below the horizon. The polar 
regions, then, have the sun six months above, and six 
months below the horizon; that is to say, they have 
one day and one night, each of six months duration, 
in a year. However, they are not, during this long 
absence of the sun, plunged in total darkness: for we 
have already seen that, independently of the twilight 
which they enjoy until the sun has descended 18° be- 
low the horizon, the moon, during this long absence 
of the sun, dispenses its light to them. We should 
add, also, that the twilight must be more intense than 
elsewhere, by reason of the rapid increase of the den- 
sity of the air, at moderate heights, in consequence of 
the almost perpetual congelation of the surface of the 
earth; and this is one of the causes which has been 
supposed to produce extraordinary refractions, in these 
regions. 



OF DATS AND SEASONS. 



267 




Finally, at the polar circles, fig. 49, the zenith very 
nearly coincides with the tropick. When, therefore, 
the sun, S, is in the plane 
of the equator and de- 
scribes the circle S E, 
which is divided by the ho- 
rizon, S' A, into two equal 
parts, the days and nights 
will be of equal length. 
But when it arrives at the 
tropick of Cancer, <£» it 
will describe the circle S" 
N, and will only graze the 
horizon with its inferiour edge, or border: consequently 
it would remain above the horizon the whole 24 hours. 
On the contrary, when the sun arrives at the tropick 
of Capricorn, 1$, it will describe the circle S' M, and, 
of course, remain the whole 24 hours below the hori- 
zon, the upper edge of the sun barely rising to the 
line of the horizon, at one point. 

We have supposed, in the foregoing explanations, 
that the sun moves round the earth, while in fact it is 
the earth that moves round the sun; but the results are 
literally the same. Nevertheless, in order to place 
side by side the apparent and real phenomena, we will 
now cause the earth to move round the sun, in speak- 
ing of the seasons. 

Let us suppose, fig. 50, S the sun, T the earth, 
S T the right .line between the centre of the sun and 
the centre of the earth, in other words the radius vec- 
tor of the sun and earth. This line touches the sur- 
face of the earth at A. All the points, then, of the 
earth, situated upon the parallel A B, will have the 
sun succesively in their zenith, as the rotary motion 
of the earth brings them, one after another, to the 
position A; and this region then has summer. If the 



268 



OP DAYS AND SEASONS. 



point A is the solstice of this season, the parallel de- 
scribed by the rotation of the earth, will be the 

Fig. 50. 

Autumn. 




Spring. 

northern tropick, and in this position the plane PTS 
is perpendicular to that of the ecliptick. But, when 
the earth, by virtue of its movement of translation, 
shall have arrived at the point directly opposite, that 
is to say, at T', the radius vector will encounter the 
surface of the earth at A', and the parallel A' B', 
which, in the preceding position, received the sun's 
rays most obliquely, will here, in turn, receive them 
vertically, and the regions thus included now enjoy 
summer, while those of the opposite tropick have 
their winter. The plane S T' P', formed by the 
radius vector and the earth's axis, is still perpendicu- 
lar to the ecliptick, as in the previous case; but the 
angle, S T P, under which the axis of the earth and 
the radius vector are joined, in the first position of the 
figure is acute, that is, it is less than a right angle, 
while that formed by the same lines, in the second 



OF DAYS AND SEASONS. 269 

position, S T ; P', is obtuse, that is, greater than a 
right angle. In the intermediate positions these lines 
form a right angle: and their angle increases from T 
to T', and decreases from T' to T. 

Finally, when the radius vector is perpendicular to 
the axis of the earth, at the points t and t', and when 
the sun appears to move in the equator, we have then 
the equinoxes; that is, the days and nights are of 
equal length throughout the earth; and the season is 
then either spring or autumn. 

The space comprised between the tropicks has re- 
ceived the name of torrid zone, because, the rays of 
the sun almost always falling perpendicularly there, the 
heat is excessive. The regions extending from the 
tropicks to the polar circles, that is, to two circles 
23 1° distant from the two poles, enjoy a more mild 
climate, and are called the temperate zones. Finally, 
the regions, almost wholly unknown, which are com- 
prised between the polar circles and the poles, consti- 
tute the frigid, or frozen zones. 

Any person may readily represent, by a very sim- 
ple experiment, the two motions of the earth, the one 
on its axis and the other in its orbit, which, by their 
combined agency, produce the phenomena of days and 
seasons. 

For this purpose bend a wire into a circle, as repre- 
sented at a b c d, fig. 51. This circle, viewed ob- 
liquely, appears elliptical. In the centre place the 
lighted candle, I: then attach a thread, K, to the pole 
of a globe that is some two or three inches in diame- 
ter. Now, if the thread be so twisted as that, by its 
untwisting, with the globe suspended by it, it shall 
cause the globe to turn from west to east, while it is 
held against the circular wire, the experimenter will 
see light and shade, or what is the same thing, day 
and night, succeed each other upon the different parts 
of the surface of the globe, in regular order, as the 
23 



270 



OF DAYS AND SEASONS. 



untwisting of the thread gives the globe a rotary motion 
upon its axis. Then, while the globe thus turns upon 

Fig. 51. 




its axis, let it be carried, by the thread, along the 
curved wire, which represents its annual orbit, its cen- 
tre being always kept on the same level as the wire, 
and the candle, which, in that case, will always be 
perpendicular to the equator, will enlighten the globe 
from pole to pole, and each part of its surface will 
successively pass through light and darkness, being an 



f 



TEMPERATURE OF THE EARTH. 271 

equal time in each. These motions, then, would 
cause a perpetual equinox; and we should always have 
days and nights of the same duration, without varia- 
tion of seasons, if the axis of the earth were perpen- 
dicular to its orbit. But this, as before shown, is not 
so. Let us, then, incline the circle which represents 
the earth's orbit, so as to give it the position A B C D, 
for example. Now if we place the globe in the 
lowest part of this circle, at Z, and while it revolves 
on its axis, by the untwisting of the thread, we move 
it along its. orbit, that is, the wire, from west to east, 
the candle will enlighten, perpendicularly, the tropick 
of Cancer, 55> and the north pole will be within the 
enlightened half of the globe. From the equator to 
the north polar circle, the days will be longer than the 
nights; while in the southern hemisphere these phe- 
nomena will be reversed. But when the movement 
of translation shall have carried the globe from H 
towards E, the border of the shadow then approaches 
the north pole, and of course it has receded, in the 
same proportion, from the south pole. The parts near 
the north pole will now be less and less enlightened, 
while those near the south pole will receive more and 
more of the sun's rays. The days, then, must de- 
crease, in length, at the north, and increase at the 
south, in proportion as the globe is carried along from 
H to E. When it is at E the candle is in the plane of 
the equator, and the border of the shadow meets that 
of the enlightened part exactly at each pole, and the 
days and nights are equal over the whole earth. 
Finally, when the globe shall have been carried to F 
and to G, we shall see reproduced, in an inverted 
order, the same phenomena which we have described. 

OF THE TEMPERATURE OF THE EARTH. 

The measures made with the micrometer and agree- 
ing with what we otherwise know of the position of 



272 TEMPERATURE OP THE EARTH. 

the earth in the ecliptick, at the different seasons of 
the year, show us that the sun is nearer to the earth, 
by /q, in winter than in summer. Yet the tempera- 
ture of summer is much higher than that of winter: and 
the pupil will therefore very naturally ask, what are the 
causes of this? There are several, of which we will 
mention only two. The first is the physical constitu- 
tion of the atmosphere, which varies materially from 
one of these seasons to the other. In summer the air 
is generally dry, but in winter it is much charged 
with vapours, which considerably enfeeble the inten- 
sity of the sun's rays. The other, and which is the 
principal cause, is, that the sun remains much longer 
above the horizon in summer than in winter. The 
night, which is the period of the loss of heat, is 
shortest in summer, when the day is longest. Some 
idea may be formed of the effect which the difference 
in the length of days and nights will produce upon 
the temperature, when we state that it has been com- 
puted that if the sun remained ten hours below the 
horizon, even in the midst of summer, ice would be 
formed, at the surface of the earth. 

Ordinarily, the temperature, in France, has been 
considered to rise from the 5th of January to the 5th 
of July, and to fall from the 5th of July to the 5th of 
January: but this scale will not, probably, be found 
strictly applicable to this country. 

The mean temperature of the equator is from 27 to 
28°. But we must remark that the southern hemis- 
phere is much colder than the northern. The reason 
of this is that the southern hemisphere is in great 
part covered by water. Now, we know that water is 
not as readily heated as the dry land, a great part of 
the calorick which it receives being constantly absor- 
bed by evaporation, congelation, and the melting of ice. 

It has also been observed that the western coasts of 
continents are much warmer than the eastern: which 



TEMPERATURE OF THE EARTH. 273 

is the effect of winds, and of the general position of 
the seas. In America, no less than in Europe, west- 
ern winds predominate. Now, these winds, coming 
from great and open seas, are always temperate; for 
the temperature of these seas is never either very high 
or very low, by reason of the mobility of the particles 
of the liquid mass, and the equilibrium they maintain 
among themselves; by which particles of water, at 
the surface, as soon as they become colder than those 
below, have so augmented in weight, relatively to their 
size, that they sink down, and warmer particles rise 
to supply their place. 

Has the earth heat properly its own? or is it de- 
pendant upon the sun for all it enjoys? These are 
questions that have engaged much attention. Some 
philosophers have advanced the latter opinion, namely, 
that all the heat of the earth is derived from the sun; 
but they are not sustained, in their position, by the 
facts in the case. We know that, at a certain depth 
below the surface, the earth remains constantly at the 
same temperature, independently of the action of the 
sun; and experiments have demonstrated that the tem- 
perature of the earth increases as we descend farther 
into it, from the surface. The ratio of this increase, 
which, indeed, is not every where the same, has been 
stated at about the average of one degree for every 
ninety feet. 

Whatever may be the cause of this internal increase 
of the temperature of the earth, whether it arises from 
original heat, in our globe, as some suppose, or from 
the incessant action of electrical and calorifick agents, 
which natural causes are perpetually mingling to- 
gether, within the earth, as others believe, we are able 
to demonstrate that this temperature has not changed, 
at least for several thousand years. Indeed, if the 
general temperature of our globe had been, at some 
ancient epoch, either higher or lower than it is at pre- 
23* 



274 TEMPERATURE OP THE EARTH. 

sent, its volume, by the effect of dilation or contrac- 
tion, must have been augmented or decreased. Now 
we know, by the length of the day, and the movements 
of the moon, that the volume of the earth has not 
changed, in the least, for the last two thousand years. 

We have seen that the temperature increases as we 
descend into the earth; but it follows the contrary 
order as we ascend above the level of the sea. In the 
ordinary state of the atmosphere we find that the tem- 
perature decreases equally with the height, if we de- 
part from the same inferiour temperature: but the law 
of progression changes with this point of departure; 
so that, in the temperate zones, for example, accord- 
ing to the observations of Saussure, it is, in winter, 
755 feet, and in summer, 525 feet, for each degree of 
the centrigrade thermometer. There is, then, a height 
where this progressive cooling attains the freezing 
point; and hence the existence of eternal snows, upon 
high mountains; and hence, also, the unequal eleva- 
tion of the points where these snows commence, in 
different climates. The vertical decrease of tempera- 
ture varies, also, with the seasons, the exposure of the 
place, and even the greater or less degree of trans- 
parency of the atmosphere. 

One of the most curious labours of the present age 
is the important application which the celebrated Hum- 
boldt has made of the geographical positions of plants 
to the measure of the mean temperature of the posi- 
tions they occupy. That remarkable traveller has 
determined, in a general manner, the elevation and 
temperature of the zones within which each plant 
seems best to thrive. Each vegetable has its proper 
and determinate limits of temperature, within which 
it is alone able to live; and the proximity of these 
limits is indicated by the stinted and imperfect vegeta- 
tion found within them. The aspect of the different 
vegetables which grow in each country, upon its plains, 



TEMPERATURE OP THE EARTH. 275 

and upon the sides of its mountains, at different ele- 
vations above the level of the sea, afford, then, a spe- 
cies of living thermometer, which indicates to the 
traveller the mean of the annual temperatures of the 
limits we have mentioned. 

As a general fact we all know that in so vast and 
moveable a mass as the atmosphere, the slightest 
causes of agitation may produce the greatest and most 
lasting commotions. It is easy to see, then, that the 
slight commotions which we daily witness, in the at- 
mosphere, are the necessary results of local causes, 
giving rise to variations in temperature; while the 
greater and more constant agitations of this elastick 
fluid are as necessarily the effect of the earth's rotary 
motion, the annual movement of the sun, and the in- 
fluence, more or less energetick, of the sun upon the 
earth, and the atmosphere, in the different seasons. 
Such are, probably, the most ordinary causes of those 
agitations in the atmosphere which we call winds, and 
which are often of long continuance. 

The most remarkable winds are those which blow 
regularly between the tropicks, and which we call 
trade winds. The following is the explanation which 
has been given of these. 

If the terrestrial globe were in a state of repose, 
and the sun always emitted its rays constantly upon 
the same part of its surface, the temperature of the 
column of atmosphere situated directly over this part, 
would become highly heated; and all the layers of air 
in this column would, in succession, one after another, 
rise to the surface of the atmosphere, like oil to the 
surface of water, or like smoke and vapour above a 
focus of strong heat; while currents of air, or winds, 
would move, constantly, from all the inferiour parts 
of the atmosphere towards this central surface. But 
the earth is constantly in motion, both upon its axis 
and round the sun; the whole equatorial zone, there- 



276 TEMPERATURE OF THE EARTH. 

fore, may be compared to the surface of which we 
have spoken, in the hypothesis of the immobility of 
the earth; for it is there that, from the beginning of 
time, the sun has constantly poured down its rays: 
there should be, therefore, as there ever has been, cur- 
rents of air rushing towards this zone, some from the 
north and others from the south. Such is the exciting 
cause of these trade winds; upon the influence of 
which navigators count with as much assurance as 
upon the periodick return of the sun, in most positions 
which are comprised between the thirtieth degree of 
latitude, north and south. 

Yet these winds do not appear to move along the 
surface of the earth in the direction of the meridians; 
that is to say, they do not appear to blow directly from 
north to south, or from south to north, as in fact, they 
really do. This arises from the rotation of the earth 
upon its axis; a movement which, taking place from 
west to east, gives to the winds from the north the ap- 
pearance of coming from the northeast, and to those 
from the south a direction from southeast. These 
appearances may be easily comprehended, by the fol- 
lowing facts: when the atmosphere is perfectly calm, 
if a person on horseback gallop the beast briskly, the 
wind will seem to blow with violence directly in his 
face. If he gallop thus toward the east, while the 
wind blows directly from the north or the south, the 
two sensations experienced are resolved into one, and 
in the former case the wind will appear to blow from 
the northeast, and in the latter, from the southeast. 
The following example also illustrates the same fact. 
Cause a globe to revolve briskly upon a vertical axis, 
and pour upon it, at the superiour pole, water, in a 
small stream. This water will not instantly acquire 
the velocity of the globe, but will tend to run down, 
by the shortest line, to the equator of the globe, or 
sphere. But the line traced upon the spherical sur- 



TEMPERATURE OF THE EARTH. 277 

face will not be a meridian line, as, if the globe were 
still, it would be, but an oblique one, which, if it were 
prolonged, would not pass through the inferiour pole. 
In like manner, it is the rotation of the earth which 
gives to the trade winds a direction towards the west, 
and not, as has been sometimes said, the attraction of 
the sun which gives rise to this phenomenon. 

We know that, at the limit or border of the region 
wherein these winds prevail, that is to say, at about 
thirty degrees either north or south, computing from 
the place occupied by the sun, these winds seem to 
blow almost directly from the east; while in proportion 
as we approach the central line they strike vessels 
more directly in a line from north to south, and from 
south to north. This effect arises from this cause, 
namely, when the cold air, rushing towards the equa- 
tor, arrives at the extreme parallels of the region in 
question, it is heated and dilated before it has acquired 
a velocity of rotation equal to the zone into which it is 
just introduced, and it moves, therefore, with less ra- 
pidity than that; and bodies situated upon this zone, 
as ships, men, &c. rush against this air, from west to 
east, with all the excess of their velocity over that 
which the air itself has acquired. From this results 
the same effect as if, the earth being immoveable, the 
wind from the east blew constantly against these bo- 
dies. But, in proportion as these columns of air move 
forward, they partake, more and more, of the velocity 
of rotation of the earth, at this part; which velocity 
they have almost completely acquired when they ar- 
rive at the central line of this zone of 60°. Hence 
it is evident that the eastern direction of these winds 
becomes less and less sensible, as we approach this 
line; and, arrived upon it, the western tendency of 
the winds is scarcely perceptible. Such, very nearly, 
would be the result from a fluid poured upon a wheel 
revolving horizontally, and which should advance gra- 



278 TEMPERATURE OP THE EAB.TH. 

duaHy from the centre towards the circumference. 
Arrived at points near the limit of this circle, it 
would not yet have acquired all its velocity; but a con- 
tinuation of the rotation of the wheel would finally 
communicate to the liquid all of its own motion. This 
liquid, than, it will be observed, would have the same 
velocity as the circumference of the wheel, while it 
would be in perfect repose, in reference to the surface 
of that circumference. Of course, we do not include, 
in this illustration, the effect of centrifugal force. 

While the dense air of the polar regions is precipi- 
tated towards the equator, to supply the void produced 
there, in the manner we have shown, thus causing the 
trade winds, those portions of the atmosphere which 
the constant action of the sun has dilated and elevated, 
near the equator, must, of necessity, in the upper 
regions of the atmosphere, form counter currents, and 
flow off, towards each of the poles, distributing their 
heat, and contributing to maintain the atmospherick 
equilibrium. The existence of these phenomena, 
first foreseen by abstract reasoning, has since been 
proved by actual observation. It has been found that 
the summit of the Peak of Teneriffe is constantly ex- 
posed to a violent wind, blowing in a contrary direc- 
tion to that of the trade winds, which raise the waters 
of the ocean against the base of the mountain. Again, 
in the year 1812, the volcanick dust and ashes ejected 
from the island of St. Vincent, passed, in thick clouds, 
over Barbadoes, to the great astonishment of the in- 
habitants. This latter island is 55 miles east of St. 
Vincent; and these ashes, after passing it, fell 50 
miles still farther east — being more than one hundred 
miles, in all, from the crater, whence they issued, and 
in a direction contrary to the violent winds encountered 
by vessels navigating the sea about these islands. In 
passing from the Cape of Good Hope to St. Helena, 
the light of the sun is often obscured, for several days, 



TEMPERATURE OF THE EARTH. 279 

by a mass of thick clouds, which are moving towards 
the south, at a great height in the air. These clouds 
are formed of the water which is raised, by evapora- 
tion, at the equator, with the heated air, and which 
has become thus condensed by the cold which it has 
encountered, in its progress towards the frigid regions 
of the southern hemisphere. 

Out of the tropicks, where the influence of the sun 
is much less than within them, the winds are occasion- 
ally subject to other causes, which, unfortunately for 
science, are not yet perfectly understood. Winds in the 
temperate climates, being much less regular, are called 
variable; nevertheless, we may regard as a general 
rule, and which is equally applicable to both, what has 
been said of the trade winds, namely, that the air, in 
moving from the north and south poles, where it was 
in repose, towards the equatorial regions, must pro- 
duce the effects of a wind from the east, or a wind in 
a contrary direction to that of our diurnal motion, 
until it has acquired, in succession, the velocity of 
each zone over which it passes; and, reciprocally, 
that the air, heated in the equatorial regions, and thus 
raised towards the superiour parts of the atmosphere, 
where it has acquired very nearly a corresponding 
velocity, must, in falling towards the poles, with that 
excess of velocity, from west to east, encounter bodies, 
at rest upon the earth, as a wind from that direction 
would do. 

The*se western winds, in many parts, out of the 
tropicks, are almost as regular as the winds within the 
tropical zone; and they have, perhaps, an equal claim 
with the others, to the name of trade winds. These 
winds abridge the duration of the passage from New 
York to Liverpool, as compared with the contrary 
passage, namely, from Liverpool to New York. Thus, 
then, in the northern hemisphere, a true north wind 
produces the effect of a northeast wind, and a true 



280 TEMPERATURE OP THE EARTH. 

south wind becomes a southwest one. England is 
exposed to these winds during three hundred days of 
the year. These phenomena, in the southern hemis- 
phere, occur in the inverted order. 

We will terminate this meteorological digression 
with a notice of two other winds which blow, with 
regularity, upon coasts, and are known under the 
names of land breeze, and sea breeze. 

Upon the setting of the sun, each day, both the 
land and the sea, which its presence has heated, lose 
their calorick, by radiation; but the loss suffered by 
the land is much more rapid, and much greater than 
that of the liquid surface. The layers of air, then, 
reposing above these two surfaces, must needs become 
unequally cooled: they do so; and that above the land, 
being colder and more dense than that over the water, 
rushes to occupy the position of that which is lighter, 
over the watery surface. This takes place in the 
morning, beginning before daylight, and continuing 
after it, until checked by the heat of the morning 
sun: it causes a wind from the land toward the sea, 
which is called the land breeze. 

But, when the sun is again above the horizon, the 
influence of his rays is soon felt; and these heat much 
more rapidly the surface of the soil than the mass of 
waters. The air, therefore, above the land is, in its 
turn, more heated and expanded than over the water. 
Towards the close of the day, therefore, when the 
inequality of temperature of these layers of atmos- 
phere has become greatest, the more dense layers 
upon the surface of the ocean are put in motion to- 
wards the land, producing the wind called the sea 
breeze. 



OF THE CALENDAR. 281 

CHAPTER SEVENTEENTH. 

OF THE CALENDAR. 

A table which indicates the division of time into 
days, weeks, months, seasons and years, is called a 
Calendar. 

The ordinary usages of civil life constantly demand 
the measure of time. We have no idea of the suc- 
cession of moments, except what we derive from 
motion; and we can only mark divisions of time by 
spaces passed over. But, in order that this may be 
exact, it is necessary that the motion by which we are 
to recognise the lapse of time, be constant and uni- 
form: and we have no such motion upon the earth. 
Man has, indeed, in himself, a principle of motion: 
his sensations and his ideas succeed each other; but 
with so much irregularity that it is not in his power 
to measure, with accuracy, the smallest interval of 
time. The mind which enjoys, and that which suffers 
acute anguish, count not the same; and time, which 
hangs heavily, in the days of. suffering and grief, flies 
with the impetuosity of youth, during the gay mo- 
ments of a joyous and agreeable life. The only con- 
stant and uniform motion with which we are conver- 
sant is that of the celestial bodies. These move with 
an unceasing and tranquil pace, through the spaces of 
the universe. Had they not motion, and had not this 
motion been observed, we could have no idea, either 
of age or duration. To man in a solitary or savage 
state, this knowledge would be of little avail; while 
it exists, in the social compact, at once the fruit of his 
industry and the evidence of his dependance. 

The interval from one rising of the sun to another, 

and which we call a day, is a division indicated by 

nature herself; but society often requires the measure 

of much longer periods of time, and these have been 

24 



282 OP THE CALENDAR. 

determined by making use of the sun and the moon. 
Indeed, the returns of the same phases of the moon, 
or of the same seasons, furnish intervals of time 
which are sensibly equal. All people have united in 
the use of these methods: some have counted by 
moons, or months; some by the revolutions of the sun, 
or years; while others, still, have made promiscuous 
application of both these methods. All this required, 
to ensure accuracy, the most exact knowledge of the 
movements of these bodies; and to those who em- 
ployed both methods of computation, the farther 
knowledge of converting either measure into the 
other, was indispensable. In this way was produced 
our present calendar, so long notably imperfect, and 
which, though often reformed, was always so difficult 
to adjust that it was ever confided, for this purpose, to 
the hands of the most able and celebrated astronomers. 

The opinion of the learned is that one of the years 
of the Egyptians — for they had several — and that of 
the Persians, had 365 days; so that every 4 years 
they lost about one day in the solar year, and after an 
interval of 1460 years, which Mas called the great 
Canicular Year, the civil and solar year again com- 
menced at the same time. The 365 days of the year 
were divided into twelve months, of 30 days each, and 
the five remaining ones were added, under the name 
of complementary days. It is this calendar which 
served as the model of that which was established by 
the French Republick. 

The Greeks had a year of 360 days, which they 
divided into 12 months of 30 days each; and every 
two years they intercalated, or added one month of 
30 days, so that they had, alternately, a year of 360, 
and one of 390 days! In this grossly erroneous way 
they continued to compute years, until about the sixth 
century before our era. At this epoch astronomical 
knowledge had made considerable progress, and from 



OF THE CALENDAR. 283 

it men had learned that the moon accomplishes its 
revolution round the earth in about 29 i days; and by- 
doubling this period they constituted two months, one 
of 30, and the other of 29 days, commencing at the 
new moon. But as twelve such months make but 354 
days, the 111 days which remained were added during 
a period of eight years, and formed three intercalated 
months of 30 days which were added to the third, 
fifth and eighth years of that period. This method of 
computation agreed very nearly with the course of the 
sun; but the Athenians, who made this reformation in 
the calendar, were apprized, by their oracle, that the 
year should be regulated by the movement of the sun, 
and the months and days by that of the moon. The 
civil year, such as they had now made it, answered 
very well to this order of the gods; but the second 
part of the order was not executed. Indeed, after one 
of these periods, the moon had still one day and a 
half to accomplish its revolution. After two of these 
periods, therefore, or sixteen years, they added three 
complementary days; and this they found to accord 
very well with the moon, but the accordance with the 
sun's motion was thereby disturbed. 

To resolve this difficulty, a celebrated Greek astro- 
nomer, by the name of Meton, who lived about 430 
years before our era, invented the cycle, or period of 
19 years, which very nearly reconciled the movements 
of the sun and moon, by embracing a complete num- 
ber of the revolutions of these two bodies. This 
period consisted of 235 lunations, namely, 228 by 
reason of the 12 lunations per year, and seven others 
for the 11 5 days by which each solar year exceeded the 
lunar on^. Of the seven lunar months, six of them 
had 30 days each, and the 7th was constituted of 29 
days. This arrangement met with so much favour 
with the Greeks that, when it came to be proposed to 
them, while assembled to celebrate the Olympick 



284 OP THE CALENDAR. 

games, it was received with acclamation, and adopted 
throughout all their dominions. The calculations of 
this cycle were exposed, in letters of gold, in the pub- 
lick places, for the use of the citizens; and it is from 
this circumstance that we have derived the name 
golden numbers, under which they still figure in our 
calendars. But after all, the cycle of Meton was not 
perfectly exact; for, after 76 years, it was found to be 
one day in advance of the course of the moon. This 
errour was afterwards corrected, by establishing a 
period of four Metonick cycles, and substracting one 
day therefrom. 

The Arabian calendar, which is that of the Maho- 
metans, is exclusively based upon the course of the 
moon, and the first day of each month is always that 
of new moon. The years of this calendar are, of 
course, very vague; and they retrograde, successively 
through all the seasons of the year. 

We pass to the Roman calendar. Of this we know 
little, before the time when Julius Cesar reformed it. 
To accomplish this, having learned, from an Egyp- 
tian astronomer, that the solar year is composed of 
365 days and I, he made the civil year to consist of 
365 clays, and added one more to every fourth year, 
to make amends for the quarter which had been neg- 
lected. This fourth year, which had 366 days, was 
called bissextile. The months, 12 in number, were 
of 30 or 31 days, except February, which had but 28 
days, except in bissextile years, when one was added, 
making the number 29. The Romans divided their 
months into three epochs, namely, the calends, which 
fell upon the first day of the month; the nones, which 
fell upon the 5th, and the ides, in like manner, upon 
the 13th. In the months of March, May, July, and 
October, the nones fell upon the 7th, and the ides upon 
the 15th day. The year determined by this calendar 
was called the Julian year. 



OF THE CALENDAR. 285 

But this year was too long by 11" 9 s ; an errour 
which amounted to about a day in 135 years: and the 
Council of Nice having, in the year 325 of our era, 
fixed the festival of Easter on the 21st of March, the 
day of the equinox, in 1582 this festival had receded to 
the 11th of the same month. To remedy this confu- 
sion, Pope Gregory XIII. published a bull or edict, 
which retrenched ten days of the year 1582; and di- 
rected that the day which, by the previous rule of 
computation, would be the 5th of October, should be 
assumed as the 15th of that month. But, in order to 
prevent a recurrence of the same difficulty, a farther 
modification was necessary. The intercalated day 
had, to this period, been regularly added to February, 
every four years: it was now ordered that in the space 
of 400 years three bissextiles should be retrenched, 
or omitted; in such a way, that, now, each year whose 
number is not divisible by 4, has 365 days, while 
every year divisible by 4, and not divisible by 100, has 
386; every year divisible by 100 and not by 400, has 
365 days, and every year divisible by 400 has 366. 
Thus the year 1600\vas bissextile, 1700, 1800, and 
1841, were neither of them so, nor will 1900 be; but 
the year 2000 will be bissextile year. The errour 
left by this correction is so small that in 10,000 years 
its accumulated quantity would be less than two days 
and fifteen hours. 

Such is the Gregorian Calendar, or neiv style, as it 
is sometimes called, which is at present adopted by 
most Christian countries. But its introduction was 
not originally effected without some difficulty. As the 
correction was made under the sanction of a Roman 
Catholick Pontiff, religious prejudices were immedi- 
ately enlisted, both for and against the measure. 
Catholick countries eagerly seized upon its adoption; 
and in these the new calendar was fully established, 
within the following year, namely, in 1583. The 
24* 



286 OP THE CALENDAR. 

Protestant states in Germany resisted the improvement 
until 1700, when they sanctioned its adoption; but the 
English obstinately persisted in crrour half a century 
longer. In 1752, however, an act of their Parlia- 
ment was passed, establishing the Gregorian calendar; 
and to introduce it, eleven days were struck out, by 
this act, which provided that the third of September, 
according to the old style, should be counted the 14th 
of that month, in the new computation. There are 
now in Europe only the Russians, and the Christians 
who adopt the rights of the Greek church, that follow 
the Julian calendar, of which the year now commen- 
ces 12 days after ours. This is the cause of the dif- 
ference which we observe between their dates and 
those employed by us; or, between old style and new. 
Months are divided into weeks. With us the week 
is of seven days; and these are Sunday, Monday, 
Tuesday, Wednesday, Thursday, Friday, and Satur- 
day. Soils, Iaiucb, Mortis, Mercurii, Jovis, Veneris, 
and Saturni, were the days of the Roman week, and also 
the names of the seven planets. So with us, Satur- 
day, Sunday and Monday clearly denote Saturn's day, 
the Sun's day, and the Moon's day; and Tuesday, 
Wednesday, Thursday and Friday, are the days of 
Tuisco, Woden, Thor, and Friga, which are the 
Saxton names for Mars, Mercury, Jupiter, and Venus. 
So the French have Lundi, the Moon's day, Mardi, 
Mars' day, Mercredi, Mercury's day, Jeudi, Jupiter's 
day, Vendredi, Venus' day, Samedi, Saturn's day, and 
Dimanche, the Sun's day. But what we should not 
have discovered, had not the historians informed us of 
it, is the order in which these planets gave their names 
to the days of the week. The ancients classed the 
planets, or rather those bodies which they considered 
such, according to the duration of their revolutions; 
thus, Saturn, Jupiter, Pvlars, the Sun, Venus, Mer- 
cury and the Moon. Now these planets thus ar- 



OF THE CALENDAR. 287 

ranged, have given their names to the days of the 
week in the order in which we now employ them. 
Thus, the first hour of Saturday, for instance, was 
consecrated to Saturn, and for this reason, gave its 
name to the day. The second hour was consecrated 
to Jupiter, the 3d to Mars, the 4th to the Sun, the 5th 
to Venus, the 6th to Mercury, and the 7th to the 
Moon; then the 8th to Saturn, again, and so on to the 
24th hour, which, by following the calculation through, 
will be found consecrated to Mars. The first hour of 
the following day, then, is consecrated to the next in 
order, which is the Sun, and the day consequently 
takes his name; the second hour being consecrated to 
Venus, and so on through the whole. By pursuing 
this calculation, we see that each day of the week 
comes thus, in its turn, to receive its name from the 
planet to which its first hour was consecrated. 

It remains for us to say a few words upon some 
terms employed in our calendars. 

The solar cycle is a period of 28 years, after which 
the d:iys of the week return, in the same order, and on 
the same day of the month, while the bissextiles suc- 
ceed each other, regularly every four years. The bis- 
sextile years, also, at the expiration of the solar cycle, 
recommence the same course, in regard to the days of 
the week upon which those of the months fall. The 
solar cycle owes its origin to the fact that the year 
does not contain an exact number of weeks, since it 
has fifty-two weeks and one day. This cycle, then, 
would be of only seven years, (because, after this time, 
the one odd day of each year would amount to a week,) 
if there were no bissextile years; but as there is one of 
these every four years, for the most part, (though not 
always,) the cycle must embrace such a number of 
years as to include seven of these, in order that the 
day added to each one of them, may constitute a week. 



288 EQUATION OP TIME. 

We have already spoken of the cycle of the moon, 
and of the golden numbers. This cycle is a period of 
nineteen years; after which the sun and the moon re- 
turn to the same relative positions, or very nearly; for 
the conjunctions, oppositions, &c. of these bodies are, 
within about an hour and a half, the same as at the 
commencement of the period, on the same days of the 
months. 

It is not until after nineteen years that the solar and 
lunar years again commence together; and the reason 
of this is, that there is, in the interval, an excess in 
the former over the latter. The number of days by 
which the solar exceeds the lunar year, in this case, 
is what we designate epact, in our calendars. 

EQUATION OF TIME. 

In ail our calendars there are always to be found 
statements of the quantity of disagreement between our 
clocks and the sun, differing with the varying seasons 
of the year. We have already seen that the earth 
does not move in a circle, but in an ellipse, and that 
the sun occupies one of its foci: from which it has been 
shown that the earth does not move with the same ve- 
locity in all parts of its orbit, or during the whole 
year. It has also been shown that the sun does not 
move in the celestial equator, which would be the case 
if the planes of the equator and the ecliptick coinci- 
ded. Did they so, then equal portions of the earth's 
equator would pass through the meridian of the sun in 
equal times, and so of the rest of the earth's surface; 
but by reason of the inclination of the ecliptick, as 
already shown, given portions of this, the sums appa- 
rent path, do not correspond with equal portions of the 
equator. 

These two causes, combined, produce a difference 
in the length of the natural or solar days, at different 



EQUATION OP TIME. 289 

periods of the year; about one half of these days being 
a fraction more than twenty-four hours long, and about 
one half a corresponding fraction less. This gives 
rise to two species of time, namely, apparent time, or 
that indicated by the shadow of the sun, upon a dial; 
and mean time, or that measured by a well regulated 
clock. The difference between these is called the 
equation. It is mean time that our clocks and watches 
are intended to keep; that is, they mark days and 
hours of equal or equated length, throughout the year. 
Now, when the natural day is more than twenty-four 
hours long it will, of course, be noon by the clock be- 
fore it is noon by the sun, and then we say the sun is 
slow of the clock, on the other hand, when the natu- 
ral day is less than twenty-four hours long, then it 
will not be noon by the clock until after it is so by the 
sun; and in this case we say the sun is fast of the 
clock. At those periods of the year when the natural 
day is just twenty-four hours long, there is perfect 
agreement in the time denoted by the sundial, and that 
by the clock. 

The following is a table of the days of the mean 
year upon which a well regulated clock will he fast or 
slow, with reference to the sun, by the quantity of one 
or more entire minutes, at solar mid-day, or noon. 



290 



EQUATION OF TIME. 



FAST OF THE CLOCK IS INDICATED BY F, AND SLOW 
OF THE CLOCK BY S. 



Month and Day. 


Minutes. 


Mnntli and Day. 


Mmulea. 


Month and Day. 


Minutes 


January. 


F. 


May. 


s. 


September. 


s. 


2 


4 


1 


3 


27 


9 


4 


5 


15 


4 


30 


10 


6 


6 


30 


3 


October. 




8 


7 


June. 




4 


11 


11 


8 


5 


2 


7 


12 


13 


9 


11 


1 


11 


13 


16 


10 


16 





15 


14 


19 


11 




F. 


20 


15 


22 


12 


20 


1 


28 


16 


27 


13 


25 


2 


November. 




February. 




30 


3 


16 


15 


1 


14 


July. 




21 


14 


21 


14 


5 


4 


25 


13 


28 


13 


11 


5 


28 


12 


March. 




22 


6 


December. 




5 


12 


August. 




1 


11 


9 


11 


11 


5 


3 


10 


12 


10 


16 


4 


6 


9 


16 


9 


21 


3 


8 


8 


19 


8 


25 


2 


10 


7 


23 


7 


29 


1 


12 


6 


26 


6 


September. 




14 


5 


29 


5 


1 





17 


4 


April. 






s. 


19 


3 


1 


4 


4 


1 


21 


2 


5 


3 


7 


2 


23 


1 


8 


2 


10 


3 


25 





12 


1 


13 


4 




F. 


16 





16 


5 


27 


1 




s. 


19 


6 


29 


2 


20 


1 


22 


7 


30 


3 


25 


2 


24 


8 


1 





APPENDIX 



The following ' 'examination," &c. was originally 
prepared for Silliman's Journal of Science; in which 
work it appeared, in 1838. 

As the mass of facts here assembled, upon the sub- 
ject under examination, and some others, collateral 
thereto, are nowhere else to be met with, in a collected 
form, and nevertheless are such as may be supposed 
of some interest to inquirers in Astronomy, yet, as the 
style and manner of their presentation, here, are both 
unsuited to the more elementary form of the foregoing 
pages, it is manifest that, in their present form, their 
incorporation there would have been impolitick, be- 
cause detrimental. The essay, therefore, is added in 
the form of an Appendix; thus excluding it from the 
elementary exercises of the pupil, while placing it 
within the reach of any and of all who may wish to 
pursue the subject of which it treats beyond the point 
at which it has been deemed advisable to dismiss its 
discussion, in the body of this work. 



APPENDIX 



EXAMINATION OF THE THEORY OF A RESISTING ME- 
DIUM, IN WHICH IT IS ASSUMED THAT THE PLANETS 
AND COMETS OF OUR SYSTEM ARE MOVED. 

In all ages, when astronomy has been cultivated, the 
opinion seems to have been entertained, in some one 
or more of its numerous forms and modifications, that 
the regions around us, beyond our atmosphere, and to 
an indefinite extent, are supplied with a rare, invisible 
medium, of unknown composition and character, in 
which all the bodies of our solar system, and perhaps 
the bodies of all other systems also, in executing the 
several motions assigned them, are necessitated to 
move. To this substance the name of ether has usu- 
ally been applied; and by this name we propose to de- 
signate it, while we examine into its history, the evi- 
dences of its existence, and its effects. The period at 
which this celestial ether was introduced into the sci- 
ence of astronomy, no less than the race of people by 
whom it was effected, is probably beyond the reach of 
inquiry: we know only that in the most remote peri- 
ods of the history of that science, we find it constitu- 
ting a prominent part of the celestial mechanism. The 
Bramins, of India, whose astronomical tables are still 
preserved to us, assumed its existence, and figuratively 
25 



294 EXAMINATION OP THE THEORY 

supposed the stars to move themselves therein, in a 
manner similar to the movement of fish in water. The 
name by which it was known to them is akash; and 
Mr. Dow, in his dissertation upon the religion of the 
Bramins, defines it to be "a celestial element, pure and 
impalpable, in which the planets move." "This ele- 
ment," he continues, according to Bedang, offers no 
resistance; so that the planets have moved uninterrup- 
tedly therein, from their first impulsion which they re- 
ceived from the hand of Brama; and they will not be 
arrested until the moment when he shall seize them in 
the midst of their course." The Chaldeans, also, held 
this opinion, and in the figurative language of the East 
were wont to represent the planets, including the sun, 
the earth and the moon, as vessels 'moving therein, 
and suited to such navigation. Alhazen, an Arabian 
optician of the eleventh century, taught the existence 
of ether, which he designated "the substance of hea- 
ven," and he supposed it situated beyond, and differing 
in character from, our atmosphere. Tycho Brahe 
reinstated the ether of the ancients in all its rights. 
But though he regarded it as existent, he denied to 
it the power of causing refraction, which he attribu- 
ted solely to the grosser vapours of our atmosphere. 
Whatever may be the difference in the natures of these 
two fluids, says he, the atmosphere so diminishes in 
density upward, that at the point where it touches the 
ether it differs little from it. Kepler, in following the 
crowd who had gone before him, revived this theory, 
in his day, and turned the substance in question to good 
account in framing some of the absurd theories which 
he put forth, along with his immortal discoveries. In 
seeking the origin of comets, he supposed them native 
inhabitants of this ether, as fishes are of the waters of 
the earth; and that God created them to inhabit the 
immense spaces of the universe, as he did whales and 
other monsters to people the vast solitudes of the ocean. 



OF A RESISTING MEDIUM. 295 

The sombre and bloody appearance which the sun 
sometimes exhibits he attributed to a coagulation of the 
ether; and when these appearances ceased, that result 
was produced by a collection of the grosser portions, 
which had distributed its transparency, and their con- 
version into comets. 

Through the long period of time embraced by these 
references, we see the existence of this fluid matter 
every where accredited; yet so vague and indefinite do 
all ideas respecting it appear to have been, that rigid 
investigation of its character or necessity seems to 
have been quite neglected; and even its practical utility, 
so far as we know, was but very limitedly considered. 
But we are now to enter upon a new era, and that a 
very important one, in the history of this fluid; for we 
are to see it elevated from the subordinate station 
hitherto assigned it, to that of a primary agent in car- 
rying out the great motions of the universe. This ap- 
plication was the offspring of the genius of Descartes. 
The conception was a sublime one which dared to iden- 
tify the law of the general movement of the universe, 
with that of the movement of terrestrial bodies: and 
this is clue to Descartes. His vortices are a bad expla- 
nation of gravity and of the system of the world, but 
they are mechanical. He discovered that the same 
mechanism moved bodies in the celestial spaces and at 
the surface of the earth; and if he was not able to 
seize this mechanism, we should not forget that this 
new and sublime thought was of his conception. Ac- 
cording to this philosopher "matter, possessed only of 
the properties of extension, impenetrability and inertia, 
was supposed to fill all space, and its parts, both great 
and small, to be endued with motion in an infinite va- 
riety of directions. From the combination of these, 
the rectilineal motion of the parts became impossible; 
the atoms or particles of matter were continually 
diverted from the lines in which they had begun to 



296 EXAMINATION OF THF THEORY 

move; so that circular motion and centrifugal force 
originated from their action on one another. Thus 
matter came to be formed into a multitude of vortices, 
differing in extent, in velocity and density; the more 
subtile parts constituting the real vortex, in which the 
•denser bodies float, and by which they are pressed, 
though not equally, on all sides. Thus the universe 
consists of a multitude of vortices, which limit and cir- 
cumscribe one another. The earth and the planets 
are bodies carried round in the great vortex of the so- 
lar system; and by the pressure of the subtile matter, 
which circulates with great rapidity, and great centri- 
fugal force, the denser bodies, which have less rapidity, 
and less centrifugal force, are forced down toward the 
sun, the centre of the vortex. In like manner, each 
planet is itself the centre of a smaller vortex, by the 
subtile matter of which the phenomena of gravity are 
produced, just as with us at the surface of the earth." 
In this system of philosoph}^, if such it may be called, 
the agenc}>" of the ether, in causing and sustaining the 
planetary motions, is indispensable; and when we con- 
sider bow universal was the belief, by all learned and 
scientifick men, in this doctrine, for more than half a 
century, we find a ready excuse for the opinion of the 
less informed upon the subject. For more than thirty 
years after the publication of Newton's discoveries, 
this absurd doctrine of vortices kept its ground in 
France, Germany, and in the universities of England 
and Scotland. It was finally driven out of the Cam- 
bridge University, in England, by a friend of Newton's 
publishing, in 1718, an edition of their Cartesian text 
book, with notes, embracing the truths which Newton 
had disclosed. These gradually undermined the doc- 
trine of Descartes, and finally caused its expulsion. 
This, however, was a work of time; and the absurdi- 
ties in question were not generally, or even in any 
considerable degree, driven from the colleges and 



OP A RESISTING MEDIUM. 297 

learned societies of Europe, before about the year 
1720*. 

When the errours of Descartes were finally re- 
moved from the schools, and from the minds of phi- 
losophers, they gave place to the Copernican system 
of the universe, as rigidly demonstrated by Newton, 
upon the basis of the laws of Kepler. By this sys- 
tem, and these demonstrations, the celestial revolutions 
are shown to be carried on independently of all 
assistance from the ether; and the agency of that 
fluid was consequently no longer demanded. But, 
though thus discarded from all participation in plane- 
tary motion, a belief in the existence of this fluid 
was still retained by Newton, who sought to employ 
it in a new capacity. "And now we might add some- 
thing concerning a certain most subtile spirit, which 
pervades and lies hid in all gross bodies; by the force 
and action of which spirit, the particles of bodies mu- 
tually attract one another at near distances, and cohere 
if contiguous; and electrick bodies operate to greater 
distances, as well repelling as attracting the neigh- 
bouring corpuscles; and light is emitted, reflected, re- 
fracted, inflected, and heats bodies." He furthermore 



* It is, then, no mors than about one hundred and twenty years 
since even the learned world became sane upon the grand outline, 
alone, of the celestial mechanism. Three of the colleges of our 
own country were founded prior to that date, namely, Harvard, 
in 1638; William and Mary, in 1693; and Yale, in 1700. At 
that early period of our history, and with the professors' chairs, 
in these institutions, generally occupied by European scholars, we 
can hardly suppose wide deviations, in the doctrines taught, from 
the received opinions in Europe; and consequently, without any 
direct proof at hand, upon this point, we from necessity infer that 
the New World has just claims to a portion of whatever of renown 
or reproach may rightfully attach to the inculcation of the Carte- 
sian doctrine of the universe, at so late a day; and that, for a 
period of eighty years, this was gravely taught and believed, at 
one, and for shorter periods at two other of the colleges of our 
infant country. 

25* 



298 EXAMINATION OP THE THEORY 

supposed that this substance is spread through all the 
heavens; and when for lack of demonstration, uncer- 
tainty arose in his mind, he thus queried: "Is not 
this medium much rarer within the dense bodies of the 
sun, stars, planets and comets, than in the empty, ce- 
lestial spaces between them? And in passing from 
them to great distances, doth it not grow denser and 
denser perpetually, and thereby cause the gravity of 
those great bodies towards one another, and of their 
parts towards the bodies; every body endeavouring to 
go from the denser parts of the medium towards the 
rarer'?" 

In 1762 the Academy of Sciences, of Paris, pro- 
posed, for a prize, the question, "Do the planets re- 
volve in a medium of which the resistance produces 
a sensible effect upon their movements?" For this 
prize M. Fabbe Bossut was the successful competitor. 
His calculations showed him that the effect of resist- 
ance, offered to the planets, would be to diminish the 
axis of their orbits, and consequently to shorten their 
periods of revolution. An acceleration in the move- 
ments of the moon had been observed, which was 
without explanation, and on applying his reasonings 
to the motions of this planet he satisfied himself that 
the observed acceleration was due to the resistance of 
ether, encountered by the moon, in traversing her 
orbit. The sum of that resistance he measured; and 
this theory being equally applicable to all the planets, 
he extended it to them all, and subjected each to the 
resisting influence of the ether. 

The tails of comets were objects of early attention; 
and it was remarked, both by Fracastor and Apian, 
that the tail of the comet of 1531, and those of two 
subsequent ones, icere all directed opposite to the sun. 
Pingre subsequently supposed that these tails are 
formed of the most subtile portions of the comet's 
atmosphere, greatly rarefied by the sun, and driven 



OF A RESISTING MEDIUM. 299 

to the side opposite the sun, by the resistance of the 
ether; aided, perhaps, by the solar rays. This direc- 
tion of comets' tails, as laid down by Fracastor and 
Apian, seems to have been very universally adopted. 
Newton says the tails of comets arise from their heads, 
and tend towards the parts opposite to the sun. Bailly 
adopts the same opinion, in strong language, namely, 
that the tails are always opposite the sun. Delambre 
is equally unreserved. He says the tail of a comet is 
always opposite the sun, or in prolongation of the 
radius vector of the sun and the comet. Laplace calls 
them trains of vapour, always situated on the other 
side of the heads of comets, relatively to the sun. 
Vince says comets are surrounded by a dense atmos- 
phere, and from the side opposite the sun they send 
forth a tail. Bonnycastle denominates them fiery tails, 
which continually issue from that side of the comets 
which is farthest from the sun. Brewster states that 
when a comet is near its perihelion, it is accompanied 
with a tail or train of light, directly opposite the sun. 
Morse avows that comets are usually attended with a 
long train of light, always opposite to the sun. Prof. 
Farrar, of Harvard, describes the trains, and adds, their 
direction is always opposite to the sun. The younger 
Herschel describes the nucleus, and adds that from 
the head, and in a direction opposite to that in which 
the sun is situated from the comet, appear to diverge 
two streams of light, constituting the tail. Sustained 
by the high standing and great numerical force of 
these authorities, the position here assumed has quite 
regularly found credence and a place in the numerous 
works of subordinate authors; insomuch that we have 
pretty uniformly recognised it in the elementary works 
upon astronomy that we have examined in the English 
language. 

The cause assigned for this direction of* comets' 
trains, by Pingre, namely, the resistance of the ether, 



300 EXAMINATION OF THE THEORY 

appears not to have found much favour in the minds 
of his successors; consequently we find, in general, 
the expression employed, namely, "impulsion of the 
surfs rays, 77 to denote both the agent and the manner 
of that agent's action, in producing this result. 

Great additional impulse has, within a few years, 
been given to the theory of a resisting medium by the 
detailed and able paper of Prof. Encke, upon the 
observed decrease of the times of revolution of the 
comet which bears the name of that astronomer. This 
paper has been translated into English, and is more or 
less extensively quoted by almost every writer who 
has employed his pen upon celestial motions, since the 
date of its appearance. The author says: "If I may 
be permitted to express my opinion on a subject which, 
for twelve years, has incessantly occupied rne, in 
treating which I have avoided no method, however 
circuitous, no kind of verification, in order to reach 
the truth, as far as it lay in my power; I cannot con- 
sider it otherwise than completely established, that 
an extraordinary correction is necessary for Pons' 
[Encke's] comet, and equally certain that the princi- 
pal part of it consists in an increase of the mean 
motion proportionate to the time." Dr.. Bowditch, by 
reference to the memoir of Encke, supposes the 
existence of a resisting medium highly probable, as 
there disclosed, in the motions of Encke's comet, in 
its successive appearances between the years 1786, 
and 1829. Arago, of the Royal Observatory, at 
Paris, in an essay, in 1832, fully recognises this 
resisting medium, on the authority of Encke, and 
dwells at considerable length upon its effects. Mons. 
Gautier assumes that, results obtained in 1828, from 
the movement of Encke's comet, accord with those 
which Encke had previously procured, and which in- 
duced him, (Encke,) in 1823 to suppose the existence 
of a medium or etherial fluid, in space, of which the 



OF A RESISTING MEDIUM. 301 

resistance, acting as a tangential force against the 
motion of the comet, would augment the power of the 
sun, and shorten the period of revolution. The 
younger Herschel refers to Encke's memoir; admits 
its conclusions, if the premises shall be found valid, 
and adds: "accordingly, (no other mode of accounting, 
for the phenomenon appearing,) this is the solution 
proposed by Encke, and generally received." Mrs. 
Somerville, adverting, also, to Encke's memoir, deems 
the existence of resisting ether rendered "all but cer- 
tain, within a few years, by the motion of comets;" 
and this insinuated negation she quite recalls some 
eight pages afterwards, by substituting the emphatick 
words, "which puts the existence of ether beyond a 
doubt." The same pen not only prophesies that by 
this resistance, comets will be finally precipitated upon 
the sun, but also that "the same cause may affect the 
motions of the planets, and be ultimately the means 
of destroying the solar system." Upon this memoir 
of Encke, theological arguments have been founded, 
having for their object to prove the destruction of the 
solar system, through the agency of this ether; and 
so certain has that result been considered, upon this 
authority, that the most positive forms of expression 
have been employed in pointing to such a consumma- 
tion. 

It is believed that we have assembled, above, the 
leading facts and arguments upon the affirmative of the 
position of a resisting medium to the planets, so far 
as to embrace all that is requisite and necessary for a 
clear understanding and subsequent impartial investi- 
gation of the question. The method of division inci- 
dent to this arrangement has been adopted in the be- 
lief that such arrangement would afford a view, more 
distinct than any other, of the entire question. We 
proceed, then, to subject the several positions and ar- 
guments to examination, in the order of their occur- 
rence. 



302 EXAMINATION OF THE THEORY 

The evidences, if any, upon which the Bramins 
and the Chaldeans founded their belief in the existence 
of this ether, not having come down to us, the reasons 
for their faith arc placed beyond investigation: nor 
are we better circumstanced in relation to the opinions 
of Alhazen, Tycho Brahe, and some others- who, 
while they supposed such ether to occupy the celestial 
regions, gave no demonstration of the fact, nor made 
application of it to any of the known purposes of the 
universe. The opinions of Kepler, upon this subject, 
may not have received less credence, in the day they 
were uttered, than did his discovery of the fundamen- 
tal laws of the celestial movements; but they were 
promptly consigned to oblivion by the subsequent reve- 
lation that comets, no less than planets, belong to our 
solar system, and move in ellipses more or less elon- 
gated, about the sun, obeying the same laws as the 
grosser planets. Of Descartes' system, and of its 
fate, we have spoken. That system was undermined 
by the discovery and application of the law of univer- 
sal gravitation; and as this ether constituted all that 
was most essential to the Cartesian doctrine, the celes- 
tial motions were no sooner found to be carried on in- 
dependently of its aid, than the whole theory was 
abandoned. Newton, himself, as we have seen, ap- 
plied this substance, under the name of "a most sub- 
tile spirit," to the production of certain results, in his 
Principles of Natural Philosophy, and again in his 
Opticks. The passages we have quoted. These po- 
sitions appear to have had their origin in a desire so 
to explain the doctrine of gravitation as to free it from 
the implied assertion that bodies act in places where 
they are not — a form of attack which the metaphysi- 
cians chose to employ against it. Yet this was but 
subjecting the question to new difficulties; as .there is 
nothing like a satisfactory explanation of gravity in 
the existence of this elastick ether. True, a fluid dis- 



OF A RESISTING MEDIUM. 303 

posed as Newton has assumed, would urge bodies in 
the direction he supposed; but what could maintain 
this fluid in the condition of its density varying ac- 
cording to the assumed law, is as inexplicable as the 
gravity it was meant to explain. The nature of such 
a fluid, if unrestrained, must be to equalise the den- 
sity of all its parts, to the destruction of this hypothe- 
sis. That Newton did not consider gravity inherent 
in matter is manifest from the passages under conside- 
ration; and he most fully states this, in words, in one 
of his letters to Dr. Bently, as quoted by Prof. Play- 
fair. Yet how he should have supposed he had 
escaped its necessity by his resort to the agency of 
this ether — since it is clearly for this purpose that he 
sought its aid — may well be deemed inexplicable. "If 
two particles of matter, at opposite extremities of the 
diameter of the earth, attract one another, this effect 
is just as little intelligible, and the modus agendi is just 
as mysterious, on the supposition that the whole globe 
of the earth is interposed, as on that of nothing, what- 
ever, being interposed, or of a complete vacuum ex- 
isting between them It is not enough that each par- 
ticle attracts that in contact with it; it must attract the 
particles that are distant, and the intervention of par- 
ticles between them does not render this at all more 
intelligible." We may close this point of investiga- 
tion, by arraying Newton against himself. Notwith- 
standing the force with which Newton supposed bodies 
to be urged by the unequal density of the ether, in 
certain directions, yet, when treating of the tails of 
comets, his language is, "from whence, again, we 
have another argument proving the celestial spaces to 
be free and without resistance, since in them not only 
the solid bodies of the planets and comets, but also the 
extremely rare vapours of comets' tails maintain their 
rapid motions with great freedom, and for an exceeding 



304 EXAMINATION OP THE THEORY 

long time." To such and kindred anomalies have the 
greatest minds been occasionally subject, in all ages. 

We have seen that, in 1762, this theory of resist- 
ance had so far commanded attention that the French 
Academy offered, in that year, a prize for the best 
examination of it; and we have also seen upon what 
evidences this prize was awarded. The results of 
the most careful modern observation, compared with 
those of a very ancient date, including some eclipses 
observed at Babylon, as early as 719, 720, and 721 
years before the Christian era, show very clearly that 
the period of the moon's revolution is shorter in mo- 
dern than in those remote ages. This acceleration, 
Dr. Halley, the English astronomer, in 1695, believed 
to exist, and declared his conviction that he could 
demonstrate the fact. A more detailed and extensive 
labour of comparison was subsequently performed by 
the Rev. Richard Dunthorne, who, in 1749, published 
its results, and verified the truth of the suspicions of 
his predecessor. It was the cause of this acceleration 
which the French Academy demanded, in 1762. 
M. l'abbe Bossut sought that cause in the resistance of 
ether; and believing he had discovered it there, he 
made such returns of his labours to the Academy, 
that the proffered prize was awarded him: nor was the 
errour into which he had fallen, discovered for almost 
a quarter of a century afterwards. In 1786, however, 
the true cause was revealed. In that year M. le Mar- 
quis de Laplace discovered both the cause and the law 
of this acceleration. He demonstrated that it is pro- 
duced by the action of the sun upon the moon; that 
it varies with the eccentricity of the terrestrial orbit, 
and consequently that such acceleration is a necessary 
result of the law of universal gravitation. In a chap- 
ter founded upon the assumed possibility of a resisting 
ethereal fluid, Laplace says: "Hence it follows, that 
the resistance of the ether can become sensible, in the 



OF A RESISTING MEDIUM. 305 

moon's mean motion only. Ancient and modern ob- 
servations evidently prove that the mean motions of 
the moon's perigee and nodes are subject to very sen- 
sible secular inequalities. The secular motion of the 
perigee, deduced from the comparison of ancient and 
modern observations, is less by eight or nine sexagesi- 
mal minutes, than that which results from the com- 
parison of the observations made in the last century. 
This phenomenon, of which no doubt can remain, 
must, therefore, depend upon some other cause than 
the resistance of ether. We have seen that it depends 
on the variation of the eccentricity of the earth's 
orbit; and, as the secular equations resulting from that 
variation satisfy, completely, all the ancient and mo- 
dern observations, we may conclude that the accelera- 
tion, produced by the resistance of an etherial fluid, 
on the moon's mean motion, is yet insensible." 
Again: "the accordance of theory with observation 
proves to us that if the mean movements of the moon 
are varied by causes foreign to the law of universal 
gravity, their influence is so small as not yet to have 
become sensible." 

The errour or Fracastor and Apian, in regard to 
the uniform direction of the tails of comets, has en- 
joyed an extent of credence not often secured to a 
false position. Although a direction nearly in pro- 
longation of the radius vector of the sun and the 
comet is net unusual for these tails, yet observations 
very early furnished exceptions enough to destroy the 
rule which has been so long adhered to in this particu- 
lar. If, as Pingre supposed, the resistance of ether 
has any agency in producing these tails, we should 
always expect them to be situated behind the nucleus, 
relatively to the comet's actual motion, without relation 
to the position of the sun; but this is not so. Indeed 
they form so many different angles, both in regard to 
the comet's line of motion, and to the relative position 

26 



306 EXAMINATION OP THE THEORY 

of the sun, that no settled fact seems deducible from 
the circumstance of their direction. Flamsted, in his 
account of a comet which he observed at Greenwich, 
in May, 1677, is at the pains to state that its tail was 
not directed in a line opposite the sun, but deviated 
therefrom at an angle of ten degrees. Hevelius, of a 
comet he observed, in 1682, says, "sometimes its 
tail was directed pretty exactly in opposition to the 
sun, as August 30, in the morning; but often with a 
considerable deviation, as is usual in most comets." 
The great comet of 1744 had, at one time, no less 
than six distinct tails, spread out like a fan. They 
were each about 4° broad; and the space between 
these several tails was as dark as the rest of the hea- 
vens. There exist other examples of the tails of 
comets which have separated into several branches. 
Newton cites two comets, the tails of which deviated 
from a right line joining the sun and comet, one ten, 
and the other no less than twenty-one degrees. The 
comet which appeared in January, 1824, besides the 
usual tail, opposite the sun, had another directed from 
the nucleus of the comet towards the sun. "The sin- 
gular form of this comet," Says the narrator, "adds 
new difficulties to the problem by which it has been 
explained, in a manner quite satisfactory, that the im- 
pulsion of the sun's rays m the principal cause of co- 
mets' tails always taking a direction opposite to the sun." 
Much that has been written upon the cause, nature and 
character of these peculiar appendages of comets, 
appears to have been based entirely upon assumed 
data. Such authority is alike unsafe and detrimental. 
The views of Arago are more sane, and therefore 
more valuable. "Kepler supposed the formation of 
the tails of comets was the result of the impulsion of 
the solar rays, which detached from the head of the 
comet the lighter portions of that body, and removed 
them to a distance beyond it. To render this expla- 



OF A RESISTING MEDIUM. 307 

nation admissible it is necessary to prove that th. so lar 
rays are endowed with an impulsive force; fo the 
most delicate experiments have hitherto failed to^ n - 
ner such force perceptible. This force shown a^d 
admitted, it will still remain to be demonstrated wl 
the tail is not always situated opposite to the sun; wh; 
there are sometimes several tails, making, one with 
another, so great angles; why they form and again 
vanish, in so short periods of time; why some of them 
have a rapid rotary motion; and finally, why some 
comets, of which the envelope seems very light and 
delicate, exhibit no trace of this appendage. A crowd 
of other theories, more or less ingenious, have been 
proposed; but they all equally fail to explain the phe- 
nomena." 

The enormous length to which these tails have some- 
times attained, has given rise to theories no less fanci- 
ful, nor yet more philosophical, respecting the conse- 
quences of such elongation. Newton supposed that 
the extremely distant portions of these tails could 
never be recalled, by attraction, to the nucleus of the 
comet, but must be scattered through the heavens, to 
be subsequently gathered to the different planets by 
attraction, and mingled with their atmospheres, to be 
there appropriated to supply the waste of matter spent 
upon vegetation, &c. Laplace, the younger Herschel, 
and some others among the moderns, have assumed, 
that portions of comets' tails are, at each revolution, 
"•scattered in space" and that, consequently, these 
bodies are continually wasting away. So indefinite a 
phrase seems not well calculated to convey any idea 
of facts; for we must suppose the matter of these 
tails, however -elongated from the nucleus of the 
comets, will stiil obey the laws of gravitation to those 
bodies, unless brought within the stronger attraction of 
some other body: and in either case no dissemination 
of matter would take place. But the diminution of 



OF THE TffEORY 

come 5 from loss of matter, b; is not 

we | sustained. It is true that Arago, in 1^3*2, fully 
co^urred in t: : nd hence advised us that in 

tJe. then approaching return of Halley's comet we must 
iOt expect to behold so brilliant a be same 

'had been at former periods of its return to th: 
But this opinion of that astronomer he did not find 
supported by the actual appearance of Halley's comet, 
5; and this fact he has promptly announced. He 
has, also, collectively presented what has come down 
to us of the apparent size, length of tail, & c, of Hal- 
> r its Various former apparitions; and 

contrasted this with the results of the careful and accu- 
rate observations upon the same body, made at various 
points, during its last appearance. At the c.: 
these b :he reader will take the trouble to 

compare what I record of the comet of h the 

circu ::' its former apparitio: ainly 

will not find in this collection of phenomena, the proof 
that Halle gradually diminishing. I will 

a matter so delicate, observations 
made ifterent periods of ill au- 

thorize any positive deduction, that which would 
distinctly result from the two pa- 

would be that the comet had merer 
during that interval. I ought to seize, with the more 
eagerness, this occasion to combat an err 
sively accredited, (a belief in the c 

of comets,) because I believe I have 
contributed to its dissemination ?J Th:- 
theory of the diminution of comets, otherv. 
to our subject, seemed demanded by the assump: 
some that matter thus lost from these hoc: 
main diffused through-the celestial regions, of course 
offering constant obstruction to the pre g otion 

of the planets and comets. How such matter is to be 
maintained in this state of diffusion, he a 



OP A RESISTING MEDIUM. 309 

we know, been explained; nor is it easy for us to con- 
ceive how the body resisted or encountered by it shall 
be prevented from appropriating it to itself, by adding 
it to its own mass.* 

Comets, from their great volumes, as compared with 
their masses, have justly been considered, of all celes- 
tial bodies, the most necessarily subject to the action 
of any resisting medium there may be in the regions 
in which they are moved. They are known to be sub- 
ject to great disturbances, in their orbits, by the attrac- 
tion of the planets of the solar system; and revolving 
as they do in ellipses of great eccentricity, many of 
these bodies having their aphelions at such immense 
distances as are not readily appreciable, by any of our 
methods of computation, their motions are much less 
subject to rigorous demonstration than those of the 
planets. t Still so much confidence had Prof. Encke 



* The following " poetical license" occurs in the younger Her- 
schel's Treatise on Astronomy, a late work, now used in some of 
the schools of this country. It contrasts very strangely with the 
really sane and valuable portions of that work, and it would hardly 
be supposed possible that it is from the same pen with these. The 
author is treating of Zodiacal light, upon which he thus fancifully 
expresses himself. "It is manifestly in the nature of a thin, len- 
ticularly-formed atmosphere, surrounding the sun, and extending 
at least beyond the orbit of Mercury and even Venus, and may 
be conjectured to be no other than the denser parts of that medium, 
which, as we have reason to believe, resists the motion of comets; 
loaded, perhaps, with the actual materials of the tails of millions 
of those bodies, of which they have been stripped, in their suc- 
cessive perihelion passages, and which may be slowly subsiding 
into the sun' ' ! 

t Too many authors of just renown, have overlooked perspicuity, 
and written vaguely, upon this point. Brewster, (Encyclopedia,) 
has not wholly escaped the charge of sacrificing philosophical accu- 
racy to euphony, in the following: "Traversing unseen the remote 
portion of its orbit, ths comet wheels its ethereal course far be- 
yond the limits of our system. What regions it there visits, or 
upon what destination it is sent, the limited powers of man are 
unable to discover. After the lapse of years, we perceive it again 
returning to our system, and tracing a portion of the same orbit 
26* 



310 EXAMINATION OF THE THEORY 

in the conclusions he had been able to draw, in the 
paper we have mentioned, that the movements of all 
these bodies which have been visible since its publica- 
tion have been observed with increased care and assi- 
duity; while the most rigid investigations of their for- 
mer movements have not been overlooked. 

According to Prof. Encke, the comet which bears 
his name, in its several revolutions, between 1786 and 
1819, exhibited a mean decrease in the times of those 
revolutions. Now, as resistance, from an ethereal 
medium, would have the effect, by diminishing the 
velocity of the comet, to lessen its centrifugal force, 
and thus force it down nearer the sun, it follows that 
precisely the result which Encke observed, would be 
the effect of such resistance. To the agency of ether, 
therefore, was this diminution ascribed, though not 
until after all other circumstances which were sup- 
posed to have had any agency in the result had been 
carefully considered. Biela's comet, or the comet of 
six years and three quarters, was also observed with 
reference to this action of resisting ether; as was, 
finally, the comet of Halley, whose last disappearance 
was in 1836, These three are the only ones, of all 
that have been seen, whose regular, periodical return 
is known, at the present day. The acceleration in 
the mean motion of Encke's comet if not due to the 
resistance of ether, is still unexplaned. Biela's comet, 
in its return, in 1832, was also retarded, "but it 
throws new perplexity upon the question of a resisting 
medium. Encke and Gauss find a diminution of nine 
tenths of a day in the observed duration of its period, 



round the sun, which it had formerly described." Now if it leave 
the sphere of our sun's attraction must it not of necessity, gravi- 
tate to some other body, and be thus prevented from ever return- 
ing? Laplace (Systeme du Monde,) has been more careful. He 
says, "innumerable comets, after having approacbed the sun, are 
elongated from it to such distances as to prove that its empire 
extends much beyond the known limits of our planetary system." 



OF A RESISTING MEDIUM. 311 

due to this resistance. \'alz, from the computations 
of Damoiseau, finds this diminution to be eight tenths. 
Prof. Santini, from his own elements finds four tenths, 
while Encke's formula and constant, for computing 
this acceleration, only accounts for a diminution of 
three hundredths of a day. The mean of the three* 
results would show that Biela's comet experiences the 
resistance of a medium twenty-five times as powerful 
as that which is encountered by Encke's comet." 
Halley's comet remains to be noticed. We have seen 
that the two above were accelerated, though very une- 
qually, the cause of which was supposed to be the 
resisting ether. But Halley's comet, in its return to 
its perihelion, in 1835, was, from some cause, detained 
beyond its time for arriving at that point — a result di- 
rectly opposite to that in the case of the other two 
bodies. "In traversing a resisting ether the comet of 
Halley would have arrived at its perihelion, in 1835, 
sooner than if moving in a void; now on the contrary, 
according to the calculations of M. Rosenberg, that 
body, by observation, was six days behind its time, 
according to the results of calculations disconnected 
from any allowance for the action of resisting ether. 
The difference, though much less, found by M. Pon- 
tecoulant, is of the same kind! Hitherto, then, the last 
appearance of Halley's comet has added nothing to 
our knowledge of the physical constitution of the ce- 
lestial spaces." 

We have said the acceleration of these bodies is 
unaccounted for: so is the retardation; but we shall 
presently see whether other agents than ether may, 
within the bounds of probability, be supposed to give 
rise to these. Clairaut, in announcing to the French 
Academy, in 1758, that the then expected return of 
Halley's comet would be retarded six hundred and 
eighteen days beyond its previous period, by the com- 
bined action of Jupiter and Saturn, adds that, "a body 



312 



EXAMINATION OF THE THEORY 



which traverses regions so elongated from the sun, and 
which escapes, for so long periods, from our view, may 
be subject to forces totally unknown; such as the action 
of other comets, or even of planets, so distant from 
the sun as to have remained hitherto undiscovered. 77 
Uranus was unknown until 1781, twenty-three years 
after this announcement; and four other planets, be- 
longing to our system, have been discovered within 
the present century — in all Jive since Clairaut penned 
his suggestion. The masses of the several planets, 
upon which so much depends in these investigations, 
appear more or less imperfectly known. Laplace 
gives the following table of them, that of the sun 
being taken for unity. 

Mercury, . imim. 

Venus, 

The Earth, . 

Mars, . 

Jupiter, 

Saturn, 

Uranus, 
Pentecoulant, from the same unit, gives the several 
masses of the same planets thus: 

Mov^nru I 



405I7T 

154936 

2546320 

ToTo7£ 
_1 

3 5 12 

T7 9T8 



mercury, 

Venus, 
The Earth, 


. 


1 9 9 70 (T 

1 

4 1839 

356354 


Mars, . 


. 


"§"6 8 0"337 


Jupiter, 
Saturn, 


: : : : 


T053,9l4 

_J 

3 5 12 


Uranus, 


. 


!_„ 

1 7 9 1 7f 


These values, says our author, appear to us the 
most exact which have hitherto been obtained of the 



planetary masses. It will be observed that these two 
tables agree only in the masses of Saturn and Uranus; 
and of these Pontecoulant says it is very probable 
they need correcting, and that observations to deter- 
mine that fact are in progress. This was in 1834. 



OF A RESISTING MEDIUM. 313 

Since that period this great geometrician has had 
cause to change his views in relation to some of these 
values. In calculating the perturbations of Halley's 
comet, he has made use of the following values, 
namely : 

Jupiter, .... T o4l,69 

Saturn, .... sjho,2 

The Earth, .... -^oo 

These values, it will be seen, do not accord with 
those in either of the above tables. In the calcula- 
tions here referred to, the action of Venus, Mercury 
and Mars was neglected as. insensible. But a German 
geometrician, Rosenberg, on the contrary, has an- 
nounced that the action of these three bodies, neg- 
lected as insensible by Pontecoulant, was sufficient to 
produce an acceleration of six days and one third in 
the return of Halley's comet. With all these uncer- 
tainties respecting the larger known planets of our 
system, we must not forget that the masses of the four 
new planets are in no degree known, beyond the fact 
that, compared with some of the older ones, they are 
very small. But still, small as they are, they are pro- 
bably capable of exercising an influence, according to 
relative position, distance, &c. upon bodies as easily 
disturbed as comets; and yet no sane attempt at a 
demonstration of the amount of such influence can 
be made, in the present state of our knowledge, for 
want of the necessary data. Brewster, in endeavour- 
ing to account for the lost comet of 1770, supposed, 
what indeed the subsequent investigations of Laplace 
have rendered wholly improbable, namely, that one of 
these new planets had arrested that body in its course, 
and added it to its own mass. We have seen that the 
mass of Uranus, as well as of other planets, is un- 
settled: the number of its satellites is equally so. 
Herschel enumerates six. Laplace says powerful 
telescopes are necessary to perceive the second and 



314 EXAMINATION OP THE THEOR\ 

the fourth, and that the published observations of Her- 
schel upon the other four are too few to determine the 
elements of their orbits, or even incontestable to assure 
us of their existence. The younger Herschel says of 
these satellites, "two undoubtedly exist, and four more 
have been suspected." 

The immense periods of time consumed by some 
comets in performing their stated revolutions, are suffi- 
cient to convince us that the space beyond the orbit of 
the most distant planet now known to us, and within 
which moving bodies gravitate to our sun, is such that 
its extent could not easily be computed by any of our 
habitual methods. Whether planets still undiscovered 
by us are revolving there, in orbits beyond that of 
Uranus, is wholly unknown to us, and this ignorance 
of ours, while it continues, must involve in uncertainty 
the movements of all such comets as have their aphe- 
lions within the regions in question. The changes in 
the form and bulk of these bodies, in calculations so 
minute as have been attempted, to establish this theory 
of resistance, deserve attention. If, as appearances 
indicate, portions of the small masses of these bodies 
are occasionally removed from the nucleus or its vici- 
nity to form the tails, which are sometimes extended to 
enormous lengths, while at others these portions of 
matter are reassembled around the nucleus, in whole 
or in part, these changes, by shifting the centre of gra- 
vity of the cometary body, must effect the action of 
foreign bodies thereon, and consequently influence the 
comet's motions. One other source of uncertainty, 
and one too which it would seem must forever remain 
such, in the movements of comets, is their action upon 
each other. To remove this source of errour no less 
would seem to be required than to identify every co- 
met belonging to our solar system; to know the mass of 
each, the elements of the orbit it describes, as well as 
the elements of all those which perturbations may 



OF A RESISTING MEDIUM. 315 

cause it hereafter to assume; and to weigh all its dis- 
turbing forces with such accuracy as to be able to de- 
termine its place, relatively to the sun and to every 
other boby, at any given point of time. May not these 
numerous and active causes very well account, not 
only for the inequalities we have observed in the mo- 
tions of comets, but even for much greater and more 
numerous ones, without the aid of a resisting medium? 
Some of these taken singly would, indeed, produce 
only slight results; but when it is considered that "in 
the immense ellipse described by a comet, the imper- 
fection of analysis obliges the geometrician to follow 
that body step by step, as it were, without once losing 
sight of it for a single moment"' throughout its revolu- 
tions, they may readily enough be supposed to cause 
greater deviations from calculated periods than "three 
one hundredths of a day, or less than forty-four mi- 
nutes in a term of six years and three quarters. Pon- 
tecoulant deems it impossible, in the present state of 
science, to determine within one or two days, the in- 
stant of the passage of a comet through its perihelion; 
so very uncertain are the elements which astronomy 
furnishes for calculating their perturbations. 

Having thus submitted the leading positions and ar- 
guments favourable to the theory of a resisting medium 
in the celestial regions, to detailed examination, the 
whole, according to the views we have taken, may be 
resolved into the following heads: 

1st. That in periods of the most remote antiquity 
there prevailed a belief in the pivsence of ether in the 
celestial regions: but the proof, if any, upon which this 
belief was founded has not been preserved to us; nor 
are we better circumstanced, in reality, with regard to 
the basis of the faith of Alhazen. Tycbo-Brahe, Kep- 
ler, cyjc. in regard to this subject: but this belief we 
must not forget, was not coupled, so far a.- we b* 
•seen, with the theorv of resistance. 



316 EXAMINATION OF THE THEORY 

2d. That when the Cartesian theory arose, this 
ether, being an indispensable agent thereof, was every 
where believed in: not, indeed, as a resisting medium, 
but as a propelling one, which carried the planets for- 
ward in their orbits: this faith came to the ground with 
the doctrine of which it formed a part. 

3d. When the laws of universal gravitation had ex- 
posed the errours of the Cartesian system, we find 
Newton still vaguely imagining of and concerning this 
substance, but in language so indistinct as not always 
to be definable; at o.:e time supposing it to be the 
cause of gravity, and at other times, by its unequal 
density, mechanically giving direction to the motions 
of the heavenly bodies: the errour of these views is 
apparent. 

4th. The ingenious arguments of Bossut, which took 
the prize of the French Academy, in 1762, were sup- 
posed to have well shown the resisting agency of this 
ether, in the acceleration of the moon : s mean motion; 
and no doubts of. the truth of this arose, until Laplace 
demonstrated that such acceleration is wholly due to 
the law of universal gravitation. 

5th. That this ether has offered to the movements 
of comets a resistance which has rendered its agency 
appreciable. If the objections that have been offered 
agiinst this are valid, they are much more than suffi- 
cient to destroy even its plausibility. 

If the conclusions at which we have arrived, then, 
be correct, we have shown that the existence of this 
ethereal medium was for a long series of years belie- 
ved in, without evidence known to us; that it has been, 
during another long series of years, even to the pre- 
sent day, accredited, also, upon different points of evi- 
dence, at different periods of time, but all which evi- 
dence has failed to sustain the fact of its existence; 
and that, therefore, to be hereafter adhered to, fresh 
evidences of its truth will be requisite to render it more 



OF A RESISTING MEDIUM. 317 

than a mere hypothesis, or gratuitous assumption: not 
that its existence has been disproved; but only that 
confirmatory evidence of that existence no longer re- 
mains. 

The predictions, therefore, that have pointed at the 
destruction of the solar system, through the agency of 
a resisting ether, may very well be discarded. Ine- 
qualities there certainly are, in the motions of the hea- 
venly bodies; but all these are confined within narrow 
limits, and they constantly oscillate around a mean 
position. This ensures the stability and duration of 
the system. Many of them, indeed, extend through 
vast periods of time, for their accomplishment; but 
they are all the necessary consequences of the ascer- 
tained laws of gravity, and can never exceed their 
known limits. They constitute, in the sublime lan- 
guage of Pontecoulant, "immense pendulums of eter- 
nity, which beat the ages as ours do the seconds!" 
27 



DICTIONARY OF TERMS. 



EXPLANATIONS OF SOME OF THE MORE COMMON 
ASTRONOMICAL TERMS. 

Aberration, an apparent annual motion in the fixed 
stars, occasioned by the velocity of light, combined 
with the real velocity of the earth in its orbit. 

Aerolite, a meteorick stone. 

Acromatick telescope, see page 29. 

Altitude, the height of the sun, moon, or stars above 
the horizon, reckoned upon a verticle circle, in de- 
grees, minutes, &c. 

Angle, the inclination or opening of two lines meeting 
in a point. 

Aphelion, the point, in the orbit of a celestial body, 
where that body is at its greatest distance from the 
sun. 

Apogee, see page 55. 

Apsides, see page 55. 

Attraction, a property of matter, by which bodies are 
made to approach each other, without any sensible 
agent to cause this. 

Axis, an imaginary line through a planet, from pole 
to pole. Plural, Axes. 

Azimuth, see page 54. 

Barometer, an instrment by which changes in the 
weight of the atmosphere are measured. 

Circumsolar stars, those stars which seem to revolve 
round either pole. 

Colures, see page 54. 

Conjunction, see page 54. 



320 DICTIONARY OF TERMS. 

Cycle, a period of time, after which the same pheno- 
mena of the celestial bodies begin to occur again, 
in the same order. 

Cycle of the sun, is a period of 28 years, which being 
completed, the days of the month return in the same 
order to the same days of the week; the sun's 
place to the same signs and degrees, in the eclip- 
tick, ccc. 

Cycle of the moon, or Metonick cycle, called, also, the 
golden number, is a revolution of 19 years, which 
being completed, the new and full moons return to 
the same days of the month, ccc. 

Declination, see page 54. 

Direct. A planet, or other celestial body, is said to 
have a direct motion, when it moves in the order of 
the signs, as from Aries towards Taurus, ccc. 

Eccentricity, see page 52. 

Ecliptick, see page 54. 

Ellipse, see page 49. 

Elongation, the angular distance of a planet from the 
sun, as it appears to a spectator upon the earth. 

Emersion, the time when any planet which is eclipsed 
begins to recover its light again. 

Equator, see page 56. 

Equinoxes, the two points where the ecliptick cuts the 
equator. When the sun is in either of these points 
the days and nights are equal — hence the name. 

Ellipticity, eccentricity, or deviation from the circular 
or spherical form. 

Focus, see page 25. 

Foci of an ellipse, see page 52. 

Galaxy, the Milky-Way, a bright zone among the srars. 

Gnomon, an instrument, or apparatus for measuring 
the altitudes, declinations, ccc. of the celestial bodies. 

Golden Numbers, a series of numbers, from one to 
nineteen, which are used in almanacks, for deter- 
mining the times of new and full moons. 



DICTIONARY OF TERMS. 321 

Hesperus, a name given to Venus when she appears 
in the evening. 

Horizon, see page 54. 

Hour circles, the same with meridians, or great circles 
which pass through the poles of the world, and are 
perpendicular to the equator. 

Immersion, the moment when an eclipse begins, or 
when a planet enters into the dark shadow. 

Inclination, the angle which the orbit of one planet 
or comet makes with that of another, or with the 
ecliptick. 

Infer iour planets, see page 48. 

Incidence of light, the angle which rays make with a 
perpendicular to the surface upon which they fall. 
Page 19, fig. 1, A B is the reflecting surface, S I 
is the incident, and I O the reflected ray. The 
angles formed, by these two rays, with a perpen- 
dicular to A B will be found equal to each other. 

Latitude, see page 54. 

Longitude, see page 54. 

Libration of the moon, see pages 110 and 111. 

Lucifer. Venus is so called when she is morning star, 
and rises before the sun. 

Magnitudes, the fixed stars are divided into eight sizes, 
or classes, and they are designated, according to 
their sizes, as of the first, second, &c. magnitude, 
the nu inhering commencing with the largest. All 
these numbers, from first to sixth, inclusive, are 
stars visible to the naked eye, but those of the 7th 
and 8th magnitudes are telescopick stars. 

Meridian, see page 55. 

Micrometer, an instrument having a system of cross 
wires or threads, moveable upon a graduated sur- 
face, and which, when attached to telescopes, is 
employed in measuring the diameters of celestial 
bodies. See page 75. 

Mean motion of a planet, is that motion which would 



322 DICTIONARY OP TERMS. 

take place if it moved in a perfectly circular orbit, 
round the sun, and had, therein, a perfectly uniform 
velocity. 

Meniscus, see page 24, figs. 7 and 8. 

Nadir, see page 55. 

Nebula, clusters of small stars which the telescope 
discloses, in different parts of the sky, and which 
have been so named on account of their cloudy 
appearance.. 

Nodes, see page 55. 

Nucleus of a comet, see page 190. 

Opposition, see page 54. 

Orbit, see page 49. 

Occultation, is when a star or planet is hid from our 
sight by the interposition of the moon, or some 
other planet. 

Parabola, see page 56. 

Parallax, see page 56. 

Penumbra, see page 224. 

Perigee, see page 55. 

Phases, the several appearances of the moon and 
planets, according as a greater or less part of their 
illuminated hemispheres is presented to our sight. 
For phases of the moon, see fig. 24, page 108. 

Planets, (primary,) are those bodies belonging to our 
solar system which regard the sun as their centre 
of motion: they are eleven in number, namely, 
Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, 
Pallas, Jupiter, Saturn, Uranus. 

Planets, (secondary,) are satellites, or moons, which 
revolve round some one of the primary planets, re- 
garding this as their centre, as the primary planets 
do the sun. Of these there are revolving round 
the earth, one; Jupiter, four; Saturn, seven; Ura- 
nus, six: total, 18. 

Poles, see page 55. 

Plane, length and breadth, without thickness. The 



DICTIONARY OP TERMS. 323 

space included within a ring is the plane of that 
ring; and so of the orbit of a planet, &c. 

Positions of the sphere, see page 56. 

Quadrature, a celestial body is said to be in quadra- 
ture when it is 90°, or k of the entire circle from 
the sun. 

Refraction, is that variation of direction which the 
rays of light suffer by passing through mediums of 
different densities. For illustration, see page 255. 

Refection of light, see page 17. 

Radius vector; an imaginary right line from the cen- 
tre of the sun to the centre of any planet, is called 
the radius vector of the sun and that planet. Al- 
though all the planets vary their distances from the 
sun, during their revolutions, and consequently 
move faster at some times than at others, yet Kep- 
ler discovered that this imaginary line always 
passes over equal areas in equal times. 

Retrograde, a planet or comet is said to have a retro- 
grade motion when it moves in a direction contrary 
to the order of the signs. See "direct." 

Sidereal, of or belonging to the stars. 

Sidereal day, the time included between two succeed- 
ing transits of the same star across the same meri- 
dian. 

Sidereal year, the time included between two succes- 
sive returns of the sun to the same star. 

Signs of the Zodiack, see page 58. 

Solstices, see page 55. 

Solsticial points, are the two signs of the zodiack, 
namely, Cancer and Capricorn, at which the eclip- 
tick touches the tropicks, and into which the sun 
enters on our longest and shortest days. 

System, a number of bodies revolving round a com- 
mon centre, as the planets and comets revolve round 
the sun. 



324 DICTIONARY OF TERMS. 

Syzygies, those points of the moon's orbit in which 
that planet is at the time of her new and full. 

Telescopick stars, those stars which are not visible to 
the naked eye, but are only to be seen through the 
aid of the telescope. 

Transit, is the passing of one celestial body before 
another, so as to render any part of the most dis- 
tant body invisible. It is also used to denote the 
passage of a celestial body across the meridian of 
a place. 

Tropicks, see page 55. 

Trajectory, is the curve described by a body in mo- 
tion. The ellipses described by the planets and 
comets, round the sun, and the curved path fol- 
lowed by a ray of light, in the atmosphere, &c. are 
trajectories. 

Translation, is used, in astronomy, to designate the 
progressive motion of a planet in its orbit, from one 
part or point to another. j 

Vertical circles, the same as Azimuth circles, or such 
as are drawn perpendicularly to the horizon. 

Vertical plane, the surface included within the circum- 
ference of a vertical circle, (which see,) and con- 
sequently a plane following the plumb line, and at 
right angles to the horizon. 

Zenith, see page 55. 

Zodiack, see page 58. 

Zones. This name is applied to the five great divi- 
sions of the globe, namely, the torid zone, which is 
comprised between the two tropicks; the two tern,' 
temperate zones, northern and southern, lying be- 
tween the northern and southern tropicks and the 
two polar circles; and the two frozen zones, northern 
and southern, lying between the polar circles and 
the poles. 



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